The hydroformylation of methyl methacrylate (MMA) was studied with the in situ formed (o-thiomethylphenyl)diphenylphosphine (SP) rhodium catalyst to establish the effect of the rhodium precursor, temperature, ligand-to-rhodium ratio, and hydrogen and carbon monoxide partial pressures on the activity and selectivity of the reaction. The performance of the SP modified reaction was compared to the performance of unmodified and triphenyl phosphine (PPh3,) modified reactions. In the case of the SP modified reaction, the rhodium precursor had a significant effect on the conversion, but only a minor effect on the selectivity. In contrast to the PPh3 Ligand, the regioselectivity of reaction did not change as the temperature and the total pressure were varied with a fixed [H-2]/[CO] ratio. Unfortunately, the deactivation of the catalyst was strong at elevated temperatures. The increase in hydrogen partial pressure increased the conversion of MMA but had no effect on the selectivity. A minimum carbon monoxide pressure was needed to achieve good chemo- and regioselectivities. The increase in catalyst concentration accelerated the rate of parallel hydrogenation.