A reactor with surface renewal, originally designed for poly(ethylene terephthalate) (PET) polymerization, was applied for poly(butylene terephthalate) (PET) polymerization. A comprehensive model including side reactions was developed and compared with the experimental results. The diffusivity of butanediol (ED) in PET melt was measured separately by desorption experiments (D-b = 1.08 X 10(6) exp(-32600/RT) (m(2)/min)). Optimum operating temperature for PET polymerization was found to be around 250 degrees C in order to avoid degradation.