The adhesion between an organic coating and a polymer substrate may depend on the diffusion of organic coating constituents into the substrate. The diffusion of chlorinated polypropylene (CPO) into polymer substrates was quantified by using radiolabelled CPO and monitoring its dissolution subsequent to its application. The CPO was H-3-radiolabelled by reduction with (NaBK4)-K-3 in tetrahydrofuran. The characterization of native and labelled CPO by size exclusion chromatography and differential scanning calorimetry showed that the reaction did not degrade the resin. Several substrates and CPO application conditions were used in order to create a wide range of CPO-substrate interfaces. The measurement of the progressive dissolution of radiolabelled CPO showed a clear-cut differentiation between the CPO bound by the substrate and the CPO simply deposited as a film, and provided a minimum estimation of the amount which was retained through macromolecular entanglement or diffusion. A larger amount of bound CPO was found in PP/EP than in PP, in agreement with a broader CPO concentration profile (several mu m) at the surface of the former, as observed by ToF-SIMS imaging.