The kinetics of hydrolysis of terephthaloyl chloride in a stirred, two-phase system n-heptane + aqueous buffer has been studied by following the rate of consumption of the acid chloride. The reaction is first order with respect to terephthaloyl chloride and the variation in rate constant with salt concentration has been examined. From studies at different temperatures it is calculated that the energy of activation is 12 kcal mole^–1 and the Arrhenius frequency factor log₁₀ A = 4.7 sec^–1. It is concluded that the rate-determining step is the homogeneous hydrolysis of terephthaloyl chloride in the aqueous phase. The results of this study are used in part 2 to correct the rates of the interfacial polycondensation of terephthaloyl chloride with piperazine.