The addition reaction between the compacted powders of succinic anhydride and p-nitraniline was studied. Both the reactants, of specific particle size, were mixed in 1:1 molar ratio and compacted. Three compaction pressures, 9.81 × 104 N/m2, 2.82 × 107 N/m2 and 5.64 × 107 N/m2, were used for the study. Conversions were measured at 90, 100 and 110 °C for various reaction times. The mole percentage conversions decreased with the increase in compaction pressure corresponding to a decrease in the initial porosity of the compact. It was also observed that the final porosity of the compact decreased with increase in reaction temperature. The phase boundary controlled model for spheres reacting from inward was found to be suitable for the evaluation of reaction rate constant. The activation energy was calculated to be 20 to 53 kcal/g mol for the temperature range studied. © 1982.