You often hear people say that -ize is American and -ise is British.
While it is certainly true that -ise has become dominant in Britain, -ize is the older spelling, and it once was dominant everywhere.
Americans have retained it, while the British have increasingly – since the mid-20th-century I think – favoured -ise .
Why, I don't know.
Last time I checked, both the (British) Oxford and Collins dictionaries gave -ize as the first spelling.
I'm British and
I always use the -ize spelling, not least because the letter z is so underused, but also because it more accurately indicates its pronunciation (and -ise is plain ugly).
english.stackexchange.com/users/1711/Alistair
The "z" is American , the "s" is British .
As an American I always use "z" for these words, and apparently many UK spellcheckers do not mark it as wrong and the Oxford English dictionary even gives "organization" as the first spelling, saying (also organisation).
Yet my (US English) Firefox marks the "organisation" variant as misspelled and suggests "organization".
My advice: unless you are writing under guidelines that suggest otherwise, use the "z" form for these words.
english.stackexchange.com/users/30/Edward Tanguay
In publishing, we usually call these -ize and -ise spellings.
Some UK publishers prefer -ize, but -ise is the predominant UK style.
See Oxford spelling on Wikipedia .
OED prefers -ize spellings.
english.stackexchange.com/users/341/Skilldrick
The US requires -ize .
The UK accepts both, but (with the OUP as the main exception) prefers -ise .
And this is one of the few cases where "international" English follows the US convention.
The ISO favours -ize spellings.
Inbuilt computer dictionaries set
to UK English rarely support -ize .
Feel free to overrule them.
english.stackexchange.com/users/1696/TRiG
That is because many spell checkers use American English.
Basically, z is used in North American English (American and Canadian), while s is used in British spellings.
You can find explanations for this on the internet — like here, for example: http://www.studyenglishtoday.net/british-american-spelling.html
Which spelling you use, depends on who your audience is.
If your writing is to be read mainly or only by speakers of American or Canadian English, use the North American spelling.
If it is not, then use the British spelling.
english.stackexchange.com/users/19290/Tristan
Apparently, if you see a process of transformation, or as Dictionary.com defines the suffix, "1.
To cause to become, resemble or agree with" the word should end with -ize, originating from the Late Latin -izāre, and the greek izein.
Or -ize is the rule if there is a smaller word prior to the suffix.
-ise is apparently from the latin past participle of words for "take", "see", and "cut" Analyse, for example, comes from (lysis) meaning dissolution.
See http://theconversation.com/its-time-to-recognize-and-internalize-the-us-suffix-ize-19828
english.stackexchange.com/users/228870/Daniel B
The New Oxford American Dictionary reports
both the words, and the spelling checker used from the Mac OS X (10.6) corrects me when I write organisation .
Considering that US English is set as first language (followed by British English), I would think that organization is the American English word (even though it is not reported by the New Oxford American Dictionary ).
english.stackexchange.com/users/252/kiamlaluno
What is with words that have forms that end both in -zation and -sation , such as localization and localisation ? Many spell checkers recommend -zation .
Are the endings "-zation" and "-sation" interchangeable?
english.stackexchange.com
<american-english><british-english><orthography><ise-ize>
english.stackexchange.com/questions/707
english.stackexchange.com/users/313/pramodc84
It appears that the endings "-zation" and "-sation" are interchangeable in the UK but not in America. You often hear people say that "-ize" is American and "-ise" is British, however, both are accepted in the UK, although "-ise" is the dominant form there. Nowadays, American uses -ize and British English uses -ise but retains -ize as an alternative.
It appears that the endings "-zation" and "-sation" are interchangeable in the UK but not in America. You often hear people say that "-ize" is American and "-ise" is British, however, both are accepted in the UK, although "-ise" is the dominant form there. The "z" is  American , the "s" is  British . Some UK publishers prefer -ize, but -ise is the predominant UK style.  The US requires  -ize .The UK accepts both, but (with the OUP as the main exception) prefers  -ise . And this is actually one of the few cases where "international" English follows the US convention.
In general,  z  is used in North American English (American and Canadian), while  s  is used in British spellings.  Some UK publishers prefer -ize, but -ise is the predominant UK style; while the US requires  -ize . Which spelling you use depends on who your audience is. Unless you are writing under guidelines that suggest otherwise, use the "z" form for these words. Which variation you choose to use will depend on your audience or the style guide you are following. However, it is generally agreed that the spelling containing a z is North American and the spelling containing a s is British.
American uses -ize and British English uses -ise but retains -ize as an alternative. The "z" is American , the "s" is British . Some UK publishers prefer -ize, but -ise is the predominant UK style.  The US requires -ize but UK accepts both, but (with the OUP as the main exception) prefers -ise. It is because many spell checkers use American English. 
The "z" is  American , the "s" is  British . Some UK publishers prefer -ize, but -ise is the predominant UK style.   The US requires  -ize .The UK accepts both, but (with the OUP as the main exception) prefers  -ise . And this is one of the few cases where "international" English follows the US convention.
The variation containing a z is generally considered North American whilst the variation containing an s is considered a British variant. Which you choose to write will depend on your audience or the style guide you are following.