The fabrication of gradient index plastic optical fiber in a closed co-extrusion process is simulated theoretically. The concentration dependency of the diffusivities of monomers in host polymer is taken into account on the basis of a modified free volume theory of the model derived is justified by fitting it to the experimental data reported in the literature, and its performance is found to be satisfactory. We show that the diffusion of a mobile phase in a polymer solution is dependent upon the structure and the concentration of polymer, and the concentration of the other mobile phase present. (C) 2003 Elsevier Ltd. All rights reserved.