Abstract: A ceramic material based on Sialon, in particular on .beta.'-Sialon is obtained by reducing an aluminosilicate precursor by means of a gas phase comprising a mixture of hydrogen and nitrogen doped with a gaseous carbon compound. The gaseous carbon compound is a gas containing carbon in combined form, e.g. an alkane, and its concentration by volume in the gas is less than 3%. The method is particularly suitable for forming Sialon-based coatings on substrates that are solid or fibrous. For composite materials constituted by a fiber preform densified by means of a matrix, the method can be used to form an interphase coating on the fibers of the preform prior to densifying the preform, or the method can be used to form a matrix that is based on Sialon.