The kinetically-controlled synthesis of the kyotorphin precursor dipeptide with immobilized alpha-chymotrypsin was used as a model system to show that diffusional restrictions, though reducing reaction rates, can benefit product formation in the case of series-parallel reactions because the competing reactions of hydrolysis of the acyl donor substrate and the peptide product are more affected by mass transfer limitations than the reaction of peptide synthesis. Six immobilized alpha-chymotrypsin catalysts were prepared with different particle diameters and enzyme loads. More than a fourfold increase in selectivity (from 10 to 45), defined as the ratio of reaction rate of peptide synthesis to the reaction rates of unwanted side product formation, was obtained in the case of the catalyst with larger particle size and higher protein load (effectiveness factor of 0.4) with respect to one with the smallest size and lower protein load that was free of diffusional restrictions (effectiveness factor close to 1).