Diffusion and reaction between phthalic anhydride and p-nitroaniline have been studied in single cylindrical pellets with their end faces in contact with each other. Assuming a second-order irreversible reaction between the reactants, the second-order velocity constants are determined as functions of particle size, compaction pressure and temperature. The activation energies required for reaction alone are computed and compared with those for diffusion. The solid-state reaction between phthalic anhydride and p-nitroaniline is confirmed to be diffusion-controlled quantitatively. © 1983.