In this study we investigated the kinetics of the coagula formation reaction of polymer latex using an extended unreacted core model. Based on the unreacted core model, the diffusion of coagulant molecules into the unreacted core was considered in this model. This phenomenon was confirmed by measuring the coagulant concentration in the unreacted core. The coagulant molecules diffused into the unreacted core, but these did not react with the emulsifier molecules because the coagulant concentration in the unreacted core did not reach the critical coagulation concentration (C-At). This model was in good agreement with the time course of the coagula formation in many conditions. The dependences of the effective diffusion coefficient on polymer latex tube radius, temperature and the coagulant concentration were also examined. The effective diffusion coefficient calculated by this model was independent of the latex tube radius and was explained by molecular diffusion. However, this was strongly dependent on the coagulant concentration. On the other hand, the actual effective diffusion coefficient was not strongly dependent on the coagulant concentration. So we measured the coagulant diffusion rate at the latex tube surface and clarified that the pseudo-steady-state approximation was not held in this reaction system.