Abstract: Disclosed is a process for the production of carboxylic acids providing esters unlikely to be hydrolyzable, which comprises reacting a polyolefin containing an isobutylene unit at a rate of 60% by weight or higher with monoxide and water in the presence of a catalyst in an amount of sulfuric acid in the catalyst of 3 moles or more with respect to mole of C.dbd.C double bonds in the polyolefin and in amounts of carbon monoxide and water of at least a mole equivalent to the C.dbd.C double bonds in the polyolefin, said catalyst being a catalyst comprising a sulfuric acid aqueous solution in concentrations of 73% to 98% by weight and a catalyst comprising a sulfuric acid aqueous solution in concentrations of 73% to 98% by weight and phosphoric acid, each having an acid strength within a range from -6 to -9.