We carried out a kinetic study on the gelation reaction of AB-type cross-end coupling of two tetra-arm poly(ethylene glycol) (Tetra-PEG) prepolymers having amine (Tetra-PEG-NH2) and activated ester (Tetra-PEG-NHS) terminal groups by ATR-IR and UV spectroscopies. The reaction rate constant for the gelation of Tetra-PEG, k(gel), was determined in aqueous solutions with varying both prepolymer volume value of k(gel) is independent of both phi and M-w, and is comparable to that of the corresponding linear-PEG system. The k(gel) value is obtained to be around 70 dm(3) mol(-1) which is much smaller than the reaction rates of typical diffusion-controlled reaction (e.g., 10(8)-10(9) dm(3) mol(-1) s(-1)) and of cross-linking photopolymerization (10(4)-10(5) dm(3) mol(-1) s(-1)). From these results, we concluded that the gelation reaction of Tetra-PEG gel is not diffusion-limited but reaction-limited process, i.e., the diffusion motion is much faster than the reaction rate. It is thus expected that Tetra-PEG prepolymer chains can diffuse in the solution during gelation process, leading to homogeneity and high-strength of Tetra-PEG gel. These discussions imply that, in order to achieve high-efficient and homogeneous gel, it is necessary to choose reaction groups so as to undergo reaction-limited reaction.