The addition of phosphite during melt processing of polyester leads to the creation of two different types of linkages : ester bonds and bonds including phosphorus atoms. The influence of different parameters on these reactions has been investigated. Experiments carried out with different grades of poly(ethylene terephthalate) show that variables such as total concentration of reactive end-groups and hydroxyl/carboxyl chain-end ratio control not only the number but also the nature of the reactions that occur. The addition of different phosphites also reveals the influence of the nature and structure of the organic groups attached to the phosphorus atom on the reaction with polyester chain ends. Finally, the mechanisms described for polyester/phosphite systems are successfully applied to acrylic polymers, broadening the scope for application of these reactions.