This paper aims at the end-capping of poly(ethylene terephthalate) (PET) chain-ends in order to introduce anhydride moieties on this polymer. An acid chloride reactive carrier of anhydride was chosen because it selectively reacts by alkoxy-dehalogenation with the hydroxyl chain ends of PET. This type of functionalization has already been described and claimed (1-3) on various polymers. However, in the present paper, a solid-liquid synthesis is reported for the first time: The reaction was performed on solid PET powder swollen in a solvent of the trimellitic anhydride chloride. This new reaction method prevents the polymer from degrading. This point is supported by parallel studies of similar reactions performed in the melt and in solution.