Several low molecular weight carboxylic acids with different substitutions were introduced into various polymer matrices : polypropylene, polystyrene, poly(butylene terephthalate), MDI-based polyurethane, and poly(vinyl chloride). These acids were then submitted to chemical treatments by sulfur tetrafluoride and ammonia. Derivatization reactions by SF4 and NH3 gave, respectively, acyl fluorides and carboxylate ions. The wavenumbers corresponding to the <(nu)over bar>(c=o) maxima of the carboxylic acids, acyl fluorides, and carboxylate ions were determined. For each one of these species, noticeable shifts were recorded that depended on the nature of the substitutents as well as on the nature of the polymer matrix. In addition, samples of the chosen polymers were exposed to photoaging. Photooxidation of these polymers is known to involve the formation of carboxylic acids. The photooxidized samples were reacted with SF4 and NH3. Their IR characteristics were determined before and after the treatments. Comparison with the results obtained with the molecular acids permitted us to specify the chemical structure of the acids formed by photooxidation and then to determine the oxidation sites on the macromolecular chains of the different polymers.