The catalytic activity and selectivity of heterodonor phosphinerhodium catalysts prepared in situ were tested in the hydroformylation of methyl methacrylate at 100 degrees C and 60 bar. Systematic variation of the heterodonor atom in the ortho position of the ligand showed that the heterodonor atom has a significant influence on the initial reaction rates and selectivities of the reaction. An exceptionally high selectivity to methyl-alpha-formylisobutyrate was observed with [o-(methylthio)phenyl]diphenylphosphine ligand, which gave a branched to normal ratio of about 27. Study was also made of the product distribution during the experiment. The decrease in the branched to normal ratio was more severe with the diphosphine ligands than with the other heterodonor ligands.