The effect of saturation of methane with water vapor on soot formation at different saturation levels has been studied on a test facility of noncatalytic high-temperature partial oxidation of methane with syngas capacity of 8 Nm(3)/h. Relationship between the synthesis gas composition and the H2O/CH4 ratio has been revealed, as well as the qualitative and quantitative relations of the formation of soot agglomerates to the degree of saturation of pure methane or natural gas with water vapor. The structural features of the soot formed have been investigated by scanning and transmission electron microscopy and X-ray diffraction analysis.