In the scope of this work, the Fickian diffusion coefficient of a non-volatile additive in the ternary system polymer/additive/solvent is determined by analysing the equalisation process of additive mass loading profiles in thin polymeric films by means of Inverse Micro Raman Spectroscopy (IMRS). The diffusion coefficients are obtained by fitting calculated to measured additive mass loading profiles. From the experiments presented in this work, for the first time the diffusion coefficient of the plasticizer triphenyl phosphate in polymeric solutions of methylene chloride and polyvinyl acetate could be determined over a wide range of additive and solvent mass loadings.