In this work, a diffusivity model based on free volume theory is presented for the simultaneous diffusion of diglycidyl ether of bisphenol A (DGEBA) epoxy and bis(p-aminocyclohexyl) methane (PACM 20) amine monomers into amorphous polysulfone (PSU). This model is expected to predict and explain the diffusion behavior of the epoxy and amine monomers into PSU during the initial time periods. The overall free volume of the polymer system is estimated using a Kelley-Bueche approximation for free volume in a binary mixture consisting of a non-reacting thermoplastic and the reacting thermoset. The fractional free volume of the thermoset is estimated by the DiBenedetto equation. The model is valid only for low epoxy-amine concentrations and degrees of cure. The diffusivity model developed here suggests that reaction reduces the species diffusivity with increasing cure from a loss of the overall fractional free volume for diffusion. Further, a model for increased epoxy diffusivity from amine-induced PSU swelling is presented and validated using data from previous studies on the single-component diffusion of epoxy into amine-swollen PSU. By combining the reaction and swelling terms with the Arrhenius epoxy diffusivity, the epoxy diffusivity expression during the simultaneous diffusion and reaction of epoxy and amine into PSU for small times, is determined. Parametric studies on the nature of diffusivity are performed to determine the influence of the various free volume parameters on thermoset diffusion, and these studies show that the thermoset diffusivity, in general, decreases with time from reaction.