A detailed chromatographic investigation has been carried out on the influence of triphenyl phosphite addition in molten poly(ethylene terephthalate) or poly(ethylene terephthalate)/poly(butylene terephthalate) blends. The observed molecular weight evolution corresponds to torque and viscosity observations and fully confirms the occurrence of high temperature reactions between polyester and phosphite. The reaction mechanisms deduced from model compound studies are successfully applied to polymers. The formation of bonds of different types is verified : phosphite reactions with hydroxyl chain ends lead to the incorporation of significant quantities of phosphorus into the polyester backbone. A high sensitivity of these phosphorus containing links towards water, phenol or m-cresol has been observed. Subsequent carboxyl chain end attack on the phosphorus linkages leads to stable ester bonds.