To make a distinction between the influence of the dielectric constant and of methanol concentration on trypsin-catalyzed hydrolysis and methanolysis at $0^{circ}C$, a model reaction of $N^u$-benzyloxycarbonyl-L-lysine p-nitrophenyl ester with water-methanol mixtures was chosen and a kinetic study done. The $k_{cat}$ values increased with methanol concentration, in a linear manner whereas $K_{M}$ values increased in a log-linear fashion. However, the $k_{cat},$_{M}$ ratio increased at lower methanol concentrations than 30% and then began to decrease at higher concentrations. The decrease in $k_{catK_M}$observed at higher than 30% methanol concentrations is attributed to the hydrophobic partitioning effect on substrate binding. On the other hand, the increase in $k_{catK_M}$ in the 0~30% methanol concentration range seems to be due to the effect of nucleophilic cosolvent on $k_{cat}$ and of the dielectric constant on $k_m$. This explanation was verified by measuring the effect of varying the dielectric constant of the medium on kinetic constants with isopropyl alcohol chemically unrelated to the enzyme reaction as the methanol concentration is maintained at a constant level. Therefore, we conclude that the effect of increasing the methanol concentration in the model reaction on the kinetic parameters $k_{cat ;and;{K_M}}$ is caused by changes in both the nucleophilicity and the dielectric constant of the medium. Based on product analysis, the increase in $k_4, k_3$by decreasing the temperature can be accounted for by the suppression of hydrolytic reactions. This observation indicates that the nucleophile is favored by low temperatures. There was no loss of trypsin activity over a 10 h period in 60% methanol concentration at $pH^＊; 5.5,; 0^{circ}C$.EX>.