The dynamic processes of the volume phase transition of pH-sensitive hydrogel wall membranes of microcapsules are much different from those of the conventional bulk gels because of a large proton reservoir inside the microcapsule wall membrane and the extremely thin unsustained gel membrane. The time course of the size S Of poly(L-lysine-alt-terephthalic acid) microcapsules has been measured in both the swelling and shrinking processes by pH jump. A lag phase and an S-shaped increase in S are observed in the swelling process, whereas a rapid decrease in S is observed in the shrinking process. To explain the characteristic time courses, a simple model is proposed on assumption of linear dependence of the rate of change in the size and in the charge density of the microcapsule membrane against deviations from the respective equilibrium values. The unique time courses of the microcapsule sizes observed are expressed by the theoretical equations. By a fitting of the observed time courses of S to the theoretical equations, two time constants are obtained, each related to the ratio of the elastic force of the gel polymer network to the frictional force and the ionization of the membrane.