Abstract: Carboxylic acid anhydrides are prepared by carbonylating a carboxylic acid ester in liquid phase, in an essentially anhydrous reaction medium including a sulfone reaction solvent, and in the presence of a catalytically effective amount of (i) nickel and an alkyl or acyl iodide promoter therefor, and (ii) a co-catalyst which comprises at least one alkali or alkaline earth metal salt, or quaternary ammonium or phosphonium iodide.The subject process is well adapted, e.g., for the preparation of acetic anhydride by carbonylation of methyl acetate.