The present invention relates to a process for the carbon-carbon double bond isomerisation of a 2-alkyl-cyclohex-3-enyl alkyl or alkenyl ketone into a mixture comprising the corresponding 2-alkyl-cyclohex-2-enyl ketones and the corresponding 2-alkylene-cyclohexyl ketones, using as catalyst a ruthenium complex obtainable by the reaction of an appropriate ruthenium organometallic precursor and an acid.