Disclosed is a carbonylation process for the production of carboxylic acids, carboxylic acid esters and/or carboxylic acid anhydrides wherein a carbonylation feedstock compound selected from one or more organic oxygenates such as alcohols, ethers, and esters is contacted with carbon monoxide in the presence of a carbonylation catalyst and one or more onium compounds. The carbonylation process differs from known carbonylation processes in that a halide compound such as a hydrogen halide, typically hydrogen iodide, and/or alkyl halide, typically methyl iodide, extraneous or exogenous to the carbonylation process is not fed or supplied separately to the process.