Ultra-high-molecular-weight poly(ethylene terephthalate) (UHMW-PET) with an intrinsic viscosity exceeding 2 dl g(-1) (M(n) approximate to 1.1 x 10(5)) was obtained by swollen-state polymerization with hydrogenated terphenyl as the solvent. The rate of swollen-state polymerization was described well by Tomita’s kinetics for a melt-phase polycondensation reaction. This suggested that there was no essential difference in the kinetics between swollen-state polymerization and conventional melt-phase polymerization. The activation energy of swollen-state polymerization obtained was 21 kcal mol(-1), similar to that of conventional melt-phase polymerization (23 kcal mol(-1)). Other polymerization conditions, that is, the particle size of the starting PET, the flow rate of inert gas and the ratio of solvent to PET, are related to the rate of swollen-state polymerization. Compared with the rate constants of conventional polymerization methods such as melt-phase polymerization and solid-state polymerization, the propagation rate of swollen-state polymerization was about three to five times faster than that of solid-state polymerization at 220 degrees C, whereas the rate constant of the degradation reaction in the swollen state was of the same order as that in the solid state. These rate effects have resulted in UHMW-PET.