The influence of triphenyl phosphite addition in molten poly(ethylene terephthalate) or poly(ethylene terephthalate)/poly(butylene terephthalate) blends has been investigated. Torque measurements during polyester processing and corresponding intrinsic viscosity values confirm an expected chain extension. Chromatographic results similarly indicate a molecular-weight increase. The precise nature of this chain extension mechanism is questioned. Chromatographic and calorimetric observations strongly suggest the formation of ester bonds and of bonds including phosphorus atoms. The enhanced reactivity of these new links towards phosphorous by-products or phenolic solvents at high temperature could explain the degradation observed in specific conditions.