Molybdenum carbide was tested as a catalyst for the partial oxidation of methane to syngas in the presence of H2S. The fresh and used catalysts have been characterized using XRD, laser Raman and SEM-TEM. It is suggested that H2S concentration higher than 0.1 wt% in the feedstock results in conversion of the carbide phase to sulfide, and also promotes the oxidation of the carbide catalyst surface; the combined effect is catalyst deactivation. At low H2S concentration in the feedstock (<0.1 wt%) the structure of the carbide catalyst was unchanged, but the rate of carbon deposition over the reactor wall and in the catalyst bed increased, again leading to the catalyst deactivation.