Reactions between model compounds were carried out at high temperatures (150-250 degrees C) in order to provide a correct simulation of the behavior of hydroxyl and carboxyl polyester chain ends in the presence of triphenyl phosphite (TPP). The reaction between TPP and alcohol (3-phenyl-1-propanol) leads to phenoxy/alkoxy substitutions on the phosphite. The elimination of phenol in open systems is considered as the driving force of this reaction. The occurrence of multisubstitutions between alcohol and phosphite is confirmed. On the other hand, carboxylic acid (4-tert-butylbenzoic acid) reacts with triphosphite to produce ester (or phenyl ester) and phosphonate. The driving force is the creation of a stable phosphoryl bond. NMR experiments lead furthermore to conclusive results concerning the higher velocity of the acid reaction with aliphatic phosphite than of the acid reaction with aromatic phosphite. In the reactions involving alcohol, acid, and phosphite, the conditions favoring ester formation are explained : The presence of TPP is seen to greatly promote the ester production.