In this work, a mathematical model for the multi-component diffusion of reacting thermosets into amorphous thermoplastics is presented for the epoxy-amine-polysulfone (PSU) system. The governing Fickian diffusion-reaction equations for the epoxy and amine are strongly coupled through the amine concentration dependence of the epoxy diffusivity and the autocatalytic reaction terms for the epoxy and amine. Expressions for epoxy and amine diffusivity were used to formulate the coupled governing equations for the epoxy and amine, and solved numerically using finite difference methods. The model predictions are coupled with experimental Attenuated Total Reflectance-Fourier Transform Infrared Spectroscopy (ATR-FTIR) and Scanning Electron Microscopy-Energy Dispersive Spectroscopy (SEM-EDS) data. It is shown that despite certain simplifying assumptions, relatively good agreement is found for the concentration distributions with time and distance, and the interphase size as a function of processing temperature.