Tertiary carboxylic acids are presently manufactured by hydrocarboxylation of branched alkenes in the presence of CO, water and a corrosive homogeneous acid. Acidic ion-exchange resins also catalyse the hydrocarboxylation reaction when operated under specific conditions. Critical factors are the use of a moderately polar, non-basic solvent and the maintenance of a very low alkanol or alkene/water concentration in the reactor. Consistently very good performances were achieved when using the carboxylic acid product as solvent and operating the reaction in a continuous stirred tank reactor (CSTR). The best resins included the Nafion NR50 as well as styrene-based sulphonated resins with high content of acid sites (>4.5 mmol/g) and moderate divinyl-benzene content (8-12%), such as the Amberlyst 15 and 36. By contrast, inorganic solid acids, such as zeolites, sulphated zirconia or heteropoly acids showed marginal activity.