The electric power consumed by hot tantalum and tungsten filaments used to dissociate hydrogen molecules into hydrogen radicals was measured at filament temperatures of 2000, 2300, and 2500 °C and hydrogen pressures from 0.5–100 Torr. The measured power consumption at pressures above 30 Torr was well represented by a model that assumed thermodynamic equilibrium between H2and H near the filament. With decreasing pressure, however, the dissociation of H2shifted from an equilibrium‐controlled regime to a surface‐reaction‐rate controlled regime. The relationship between the power consumption and the pressure in the surface‐reaction‐rate controlled regime was correlated with the surface dissociation probability, which was determined to range from 0.18 to 0.94. ©1995 American Institute of Physics.