It is shown that lattice parameter changes that reflect the composition of the unpolymerized material occur during the solid-state polymerization of methacrylamide in solid solution with isobutyramide. In addition, phase changes also occur as is required by the phase diagram of the system. These results indicate that annealing and recrystallization must play an important role in this reaction and probably in other solid-state polymerizations. It is also likely that these effects are among the parameters that are important in determining the rate of solid-state polymerization.