p-Substituted phenylmaleimides were synthesized and their hydrolytic rates were measured by dual-wavelength method. Hammett plot for the hydrolysis of p-substituted phenylmaleimide yields a straight line of p=0.35 with relative coefficiency r=0.99. The half-life times of a series of maleimide derivatives including the practical fluorescent thiol reagents such as N-[p-(2-benzimidazolyl) phenyl] maleimide, N-(1-anilinonaphthyl-4)-maleimide and N-(7-dimethylamino-4-methyl-3-coumarinyl) maleimide were compared and their stabilities in aqueons media were discussed.