A process for the manufacture of 3-hydroxy-2,6,6,-trimethyl-2-cyclohexen-2-one, an intermediate in canthaxanthin synthesis, by isomerizing 1,1,3-trimethyl-7-oxabicyclo[4.1.0]heptan-2-one in the presence of a palladium(O) complex of an acetone derivative of the formula ##STR1## wherein R.sup.1 and R.sup.2 are aryl and R.sup.3 and R.sup.4 are hydrogen or monovalent hydrocarbon groups optionally carrying a carbonyl function, a bis-(diphenylphosphino) derivative of the formula(R.sup.5).sub.2 P--R.sup.6 --P(R.sup.5).sub.2wherein R.sup.5 is phenyl or tolyl and R.sup.6 is a C.sub.2 --C.sub.4 -alkylene group optionally substituted with one or more inert organic groups, and a triarylphosphine of the formulaP(R.sup.7).sub.3wherein R.sup.7 is an optionally alkyl-substituted phenyl group.