Multi-component diffusion of a diglycidyl ether of bisphenol A (DGEBA) epoxy and an aliphatic diamine, bis(p-aminocyclohexyl)-methane (PACM 20) into amorphous polysulfone (PSU) using a Fourier Transform Infrared Spectroscopy-Attenuated Total Reflectance (ATR-FTIR) technique has been studied, Results indicated that the diffusion of the larger epoxy molecule in polysulfone occurred more quickly when diffusing in the presence of the amine, This behavior and slight deviation of the experimental data from classical Fickian kinetics for amine diffusion into polysulfone were attributed to a finite amount of polysulfone swelling. The diffusion of a model end-capped (non-reactive) epoxy into a polysulfone film saturated with amine was studied to quantify thr effect of the amine-induced polysulfone swelling on the diffusivity of the second component. In the DGEBA/PACM 20 system, simultaneous diffusion and reaction in PSU was quantified. Reaction rates in the PSU did not significantly deviate from intrinsic reaction rates of the bulk material. SEM studies of etched epoxy-amine-PSU interphases reveal a semi-interpenetrating network of cured epoxy-amine ranging from 0.6 to 2.4 mu m, depending on the processing temperature. The formation of this interphase structure is the primary method of load transfer between these dissimilar materials. This work demonstrates the diffusion, subsequent reaction, and final phase separation as the mechanism for interphase formation in the polysulfone/epoxy-amine system studied,