In an attempt to tailor low-cost, precious-metal-free electrocatalysts for water electrolysis in acid, molybdenum carbide ( beta -Mo sub(2)C) nanoparticles are prepared by in situcarburization of ammonium molybdate on carbon nanotubes and XC-72R carbon black without using any gaseous carbon source. The formation of Mo sub(2)C is investigated by thermogravimetry and in situX-ray diffraction. X-ray absorption analysis reveals that Mo sub(2)C nanoparticles are inlaid or anchored into the carbon supports, and the electronic modification makes the surface exhibit a relatively moderate Mo-H bond strength. It is found that carbon nanotube-supported Mo sub(2)C showed superior electrocatalytic activity and stability in the hydrogen evolution reaction (HER) compared to the bulk Mo sub(2)C. An overpotential of 63 mV for driving 1 mA cm super(-2) of current density was measured for the nanotube-supported Mo sub(2)C catalysts; this exceeds the activity of analogous Mo sub(2)C catalysts. The enhanced electrochemical activity is facilitated by unique effects of the anchored structure coupled with the electronic modification.