Thin layers of a-Si3N4 were synthesized by the pyrolysis of thin films of poly(methylsilane) (PMS) and poly(dimethylsilane) (PDMS) spin-coated on silicon single-crystal wafers and via deposition of the volatile species resulting from the thermal cracking of the bulk precursor in the presence of ammonia. The process was monitored by FT-IR spectroscopy. The reaction between NH3 and PMS begins at 200 °C with the slow production of a slightly cross-linked product involving Si3N knots. Extensive amination of PMS occurs on pyrolysis at 300 °C, under 5−10 Torr NH3 overpressure. The product exhibits IR bands characteristic of both a silazane and an aminosilane species, which are presumably formed by Si−H and N−H heterodehydrocoupling. Between 200 and 450 °C, this cross dehydrocoupling reaction competes very effectively with the Kumada rearrangement. Significant loss of carbon occurs from the resulting poly(carbosilazane) between 500 and 600 °C. Prolonged curing under NH3 at 300 °C, to remove all Si−H groups and to give a densely cross-linked polysilazane, suppresses the Kumada rearrangement, and negligible carbon loss occurs on raising the pyrolysis temperature to 700 °C. Although the main product is still a-SI3N4, there is an increased amount of residual carbon.