The ternary Nowotny phase (NP), with a composition Mo3+2xSi3C0.6 (x = 0.9-0.764), is found to be catalytically active in the field of electrochemical water splitting. The NP embedded in a porous SiC/C nanocomposite matrix is synthesized via a single-source-precursor approach which involves the reaction of allylhydridopolycarbosilane with MoO2(acac)(2). Thermal treatment of the single-source-precursor up to 1400 degrees C in a protective atmosphere results in the in situ formation of nanocrystalline Mo3+2xSi3C0.6 immobilized in a thermally and corrosion-stable SiC/C matrix. The weight fractions of the observed crystalline phases Mo3+2xSi3C0.6 and SiC amount to ca. 28 (26) and 72 (74) wt%, respectively, when prepared at 1400 degrees C (1350 degrees C). The porosity of the formed nanocomposite is adjusted by the addition of polystyrene (PS) as a pore former to the single-source-precursor resulting in a specific surface area up to 206 m(2)/g. The electrocatalytic activity of the Mo3+2xSi3C0.6/C/SiC nanocomposite with respect to the hydrogen evolution reaction (HER) is characterized by low over potentials of 22 and 138 mV vs reversible hydrogen electrode (RHE) for applying 1 and 10 mA cm(-2) of current density, respectively. The analyzed electrocatalytic performance exceeds that of most Mo-based electrocatalysts and shows high stability (over 90%) during 35 hours.