Diffusion from one layer of a plasticized polymer film to the adjacent laminated layer is examined. Considering the importance of having a non-destructive, real-time method of monitoring interfacial diffusion, we developed a new spectroscopic technique. In the presented method, one of the two layers of a laminated polymer film is doped with electron-acceptor molecules, 2,4,7-trinitro-9-fluorenone, while the other layer contains a polymer with electron-donating side chains, poly(N-vinylcarbazole). As electron-acceptor molecules migrate to the adjacent layer, a charge-transfer complex is formed. The charge-transfer complex has a distinct optical absorption band, which is used in the presented technique to monitor its formation. The kinetics of charge-transfer complex formation is diffusion-controlled, and thus it is used to deduce diffusion coefficients of the mobile electron acceptor in various polymer matrices. The implementation of this technique in monitoring other transport processes is considered as well. Detailed mathematical modelling of the interlayer diffusion process based on the obtained experimental data is presented. Diffusivities found for 2,4,7-trinitro-9-fluorenone tracer in cellulose acetate butyrate, poly(vinyl acetate) and poly(vinyl butyrate) polymer matrices were 5 x 10(-9), 2.5 x 10(-9) and 2.5 x 10(-9) cm2 s-1, respectively. Diffusivities increase as diffusion from the laminated layer progresses owing to plasticizing of the matrix by the diffusing molecules.