A catalyst useful for carbonylation of olefins has been developed. The catalyst comprises a palladium compound, e.g. PdIm.sub.4Cl.sub.2, where Im is imidazole and HCl dissolved in water or an alcohol. Carbonylation using this catalyst involves contacting an olefin stream preferably in a solvent such as o-xylene with the catalyst and carbon monoxide at carbonylation conditions to provide a carboxylic acid or an ester. When the catalyst solvent is water one can obtain an acid as the product, but when the catalyst solvent is an alcohol one obtains an ester as the product.