A method for the manufacture of the four geometric isomers of 4(2-butenylidene)-3,5,5-trimethyl-2-cyclohexen-1-one is provided. The process comprises pyrolyzing a compound of the formula ##STR1## wherein R represents lower-alkyl-oxycarbonyl, aryl-oxycarbonyl, or, preferably, lower-alkanoyl, benzoyl or substituted benzoyl.