Abstract: A process for the preparation of beta-disubstituted monocarboxylic acids derived from tertiary acyclic, monocyclic or bicyclic hydrocarbons. The process consists in reacting said hydrocarbons with vinylidene chloride and either a functional compound where the molecule includes a tertiary carbon atom bonded to the heteroatom of a functional group such as a tertiary alcohol, an ether, or a tertiary halide or an alkene, in the presence of a concentrated protonic acid. The reaction can be carried out in the presence of a catalyst of the Lewis acid type and particularly BF.sub.3. These acids are useful as synthesis intermediates and as additives for oils and lubricants.