A series of alpha-, beta,beta-, and alpha,beta,beta-deuterium-labeIled acrylonitrile monomers were prepared and polymerized. Secondary deuterium isotope effects on the polymerization and on the pyrolysis reactions that precede carbon fiber formation were observed. When deuterium is in the alpha-position, the polymerization rate is greater and the molecular weight is higher. It is proposed that either the propagation rate constant or both that constant and the termination rate constant are increased on deuterium substitution. In differential scanning calorimetry, the polyacrylonitrile exotherm occurs at higher temperatures and is narrower when deuterium is substituted at the alpha-position. On the other hand, the thermal gravimetric analysis activation energy for weight loss of polymer at temperatures below the acrylic exotherm is lower when deuterium is in the a-position, relative to the alpha-hydrogen polymers. As there is no correlation between the weight loss energy of activation and the various exotherm parameters, the weight loss and the exotherm are considered to be independent events. Examination of the distribution of deuterium substituted ammonia species evolved during 100-240 degrees C thermal treatment of the alpha- and beta,beta-deuterated polyacrylonitriles provides a clear indication that both the alpha- and beta-positions are directly involved in hydrogen migration to nitrogen, but the mechanism of ammonia generation remains unclear.