The role of hydrogen in the direct nitridation of silicon was studied in conjunction with the role of oxygen contained in both the gas and solid phases at temperatures ranging from 1150 to 1350 degreesC in nitrogen containing hydrogen from 0 to 10 vol %. Hydrogen prevented a protective layer, being composed of silicon dioxide, silicon oxynitride, or a mixture of these two, from forming via the reaction of silicon with impurity oxygen contained in the nitrogen. The protective layer which had once formed in the absence of hydrogen hindered the subsequent nitridation even in the presence of hydrogen. The native oxide covering the silicon surface not only inhibited the nitridation of silicon but also kept the protective layer from forming. Silicon could react with nitrogen only after the removal of native oxide and in the presence of hydrogen at a concentration as low as 0.3 vol %.