We report results of experiments in which samples of ultramafic xenoliths from the Udachnaya-Vostochnaya kimberlite pipe (Siberian craton, Yakutia) were heated, in a flow reactor, by flows of reduced gas at temperatures between 800 °C and 1200 °C. The heating tests were preceded by investigation into the gas composition of main phases. When subjected to heating by the reduced gas flows, the samples of metasomatized lithospheric mantle released reduced fluids and formed partial melts, the composition of the latter being controlled by fusibility of rocks (“readiness” of liquid to segregate). The initial compositions of the partially molten rocks turned out to correlate with those of secondary inclusions in matrix minerals and of most fusible minerals in reaction rims and in metasomatic veins, as inferred from the analysis of vitrified melting products and structure of the heated rocks. The effect of partial melting of matrix minerals was to increase the contents of Al (melting of garnet) and Mg (olivine and orthopyroxene rims) in the melts. The experimental melts are difficult to compare with natural felsic and mafic rocks in average compositions. The bulk compositions of fluids in altered ultramafics disagree with the computed equilibrium compositions according to some correlation relations (CO2 ↔ CO, r+).