most preferred z group is a phosphate ester particularly one bonded to a phenyl chromophore group y through an oxygen atom the phenyl group in turn being bonded to the 4 carbon atom of the dioxetane ring i e str3 the phosphate ester is cleaved by alkaline or acid phosphatase enzymes table 1______________________________________group z enzyme______________________________________ 1 str4 alkaline and acid phosphatases 2 str5 esterases 3 str6 decarboxylases 4 phospholipase d str7 5 str8 u03b2 xylosidase 6 str9 u03b2 d fucosidase 7 str10 thioglucosidase 8 str11 u03b2 d galactosidase 9 str12 u03b1 d galactosidase10 str13 u03b1 d glucosidase11 str14 u03b2 d glucosidase12 str15 u03b1 d mannosidase13 str16 u03b2 d mannosidase14 str17 u03b2 d fructofuranosidase15 str18 u03b2 d glucosiduronase16 trypsin str19 17 trypsin str20 ______________________________________ suitable x groups are described in the summary of the invention above preferably x contains one or more solubilizing substituents i e substituents which enhance the solubility of the dioxetane in aqueous solution examples of solubilizing substituents include carboxylic acid or carboxylic acid salt groups e g carboxymethyl or carboxyethyl groups which are most preferred sulfonic acid or sulfonic acid salt groups e g oxysulfonylmethyl and quaternary amino salts e g trialkylammonium bromide preferably the enzyme which cleaves group z is covalently bonded to a substance having a specific affinity for the substance being detected examples of specific affinity substances include antibodies e g anti hcg antigens e g hcg where the substance being detected is an antibody e g anti hcg a probe.