in the initial feed is less than about 10 wt only trace amounts of tri substituted olefins have been found in the isomerized product when the amount of vinylidene olefin in the initial olefin feed exceeds about 37 wt amounts of tri substituted olefin up to about 14 wt but generally in the range of 1 3 wt have been found in the isomerized product the isomerization can be conducted by placing the olefin feed in a pressure reaction vessel and adding a small amount of an iron carbonyl catalyst and heating the mixture preferably under an inert atmosphere such as nitrogen to an isomerization temperature alternatively the olefin feed containing a catalytic amount of an iron carbonyl can be passed through a continuous reactor such as a tubular reactor maintained at isomerization temperature no additional stimuli such as ultraviolet light or ultrasonic energy is needed to activate the iron carbonyl catalyst any iron carbonyl catalyst can be used such as fe co 5 fe 2 co 9 fe 3 co 12 and the like in the examples the catalyst added to the olefin mixture was fe co 5 only a catalytic amount of the iron carbonyl is needed a useful range is about 0 01 3 0 wt a preferred range is about 0 02 1 0 wt the olefin containing the iron carbonyl catalyst is heated to isomerization temperatures this is a temperature high enough to cause the isomerization reaction to proceed but not so high as to cause excessive decomposition of the olefins a useful temperature.