The reactions occurring between polyesters and phosphorus compounds in the molten state are investigated. A large molecular weight increase is observed in blends of Poly(ethylene terephtalate) and Poly(butylene terephtalate) after triphenyl phosphite addition. The formation of links containing phosphorus atoms between the polyester chain ends has been established. The weakness of this bond makes the observations difficult. An effect of phosphite on promoting the direct esterification between chain ends is suggested but not definitely proved yet.