By investigating the nitridation processes using a large mass of silicon under different atmospheres, it was found that the nature of heat conduction and mass transport were influenced by the size of the silicon powder compact and/or the number of compacts. Two main reaction maxima were observed and both accelerating as well as retarding influences on the reaction rate could be identified. The extent and starting temperature of the first reaction maximum was found to be greatly influenced by gas composition. Also investigated was the influence of different nitridation atmospheres (N2+Ar, N2+H2, N2+H2+Ar and N2+He) on microstructure of large silicon powder compacts. A homogeneous and complete reaction could be achieved by tailoring the gas composition