Hydrocarbons were decomposed on the surface of an incandescent tungsten filament in a low pressure flow reactor. The rate of conversion of tungsten to ditungsten carbide W₂C was measured by a resistance method. Carbon accumulation on the outer W₂C surface was prevented by rapid inward diffusion. The sticking probability γ was defined as the probability that a room temperature hydrocarbon molecule striking the hot W₂C surface would undergo complete decomposition to carbon and hydrogen. The value of γ for different hydrocarbons, measured over a 300-fold range of conversion rates, increased in the order: methane<propane≃cyclopropane<neopentane. Mixing of hydrogen gas with the hydrocarbon reactant markedly increased γ. For example, at a filament temperature of 2610°K, γ increased from 6.4 × 10^–3(methane only) to 2.6 × 10^–2(methane plus hydrogen). This novel effect is explained by carbon patches on the W₂C surface. These patches are associated with low values of γ but can be removed by adsorbed hydrogen.