The process of carbothermal reduction of SiO2 in a nitrogen flow at temperatures of 1673-1723 K was investigated. It was established that mixtures obtained by the sol-gel technique are not microhomogeneous and consist of silica and carbon aggregates, inside which the processes of structure and phase ordering occur during heat treatment. The contact region of these aggregates is the place where SiO2 reduction occurs. In this region the following transformation takes place : partial SiO2 reduction, appearance of a film of melt of the composition SiOx, where x < 2, enveloping of the carbon particles by this melt, destruction-activation of the carbon particles, implantation of nitrogen in the highly oxygen-defective melt, formation of silicon oxynitride and, subsequently, silicon nitride. alpha-Si3N4 forms in the presence of a large carbon nucleus, beta-Si3N4 forms in its absence (or on two-dimensional particles).