The chemical modification of PVC films with sodium azide and 4-aminothiophenol was investigated. Using mixtures of a good and a nonsolvent for PVC, reactions took place in a controlled way, and the films remained transparent and smooth. To obtain information about the surface selectivity of the reactions, the degree of modification of the polymer across the film was determined by depth profiling carried out using confocal Raman microscopy. It is shown that the reaction with sodium azide always occurs homogeneously through the film while in the case of aminothiophenol, conditions could be found that allowed the polymer film to be preferentially modified at the surface. The gradient of the degree of modification and the surface selectivity depend on the relationship between the rate constants of the substitution reaction and the diffusion process of the reactant. They are, therefore, functions of reaction time, temperature, and the proportion of solvent/nonsolvent used for the reaction. The measured depth profiles as a function of reaction time have been fitted to a solution of the second Fickian law and combined diffusion/reaction coefficients for the transport of the reactant have been calculated.