Methacrylamide polymerizes in the solid state in a manner analogous to acrylamide; i.e., the polymer nucleates as a second phase during reaction. However, in this case, the reaction is sensitive to the presence of reactive gases. These gases are presumed to diffuse along surfaces, lattice defects, and the monomer-polymer interface. Nitric oxide and sulfur dioxide inhibits both the in-source and postirradiation polymerization. Oxygen behaves anomalously. It inhibits the postirradiation reaction, but accelerates the in-source one. It apparently does this by shortening the induction period. A possible mechanism is suggested.