The influence of relative humidity (RH), between 55 and 100%, on the rate of hydrolytic degradation of polyarylate was investigated at 100°C. The equilibrium water sorption in weight percent, C∞, was determined to be C∞ = 0.0043 X (RH)1.2. For specimens 0.32 cm thick, it takes about 24 h to reach 0.9C∞ at 100°C. The decrease in molecular weight (M̄w) due to the hydrolytic attack of water was linear with time, suggesting a zero-order process with respect to M̄w. The rate of decrease was found to be proportional to C∞. A 30% decrease in M̄w was observed after 21 days at 55% RH and 100°C and, at that point, the material has become brittle.