The H−negative ion thermal energy measured using the two laser‐pulse photodetachment technique is reported to be in the range from 0.1 to 0.7 eV for various conditions of volume ion source operation (pressure–from 2 to 7 mTorr, discharge current–from 1.5 to 20 A). The hydrogen pressure has a significant effect in lowering the negative ion temperature, while the increase of the discharge current leads to a raise in T−. It is found that T−represents a fraction of the electron temperature,Te, which is approximately independent of the discharge current, but strongly depends on the gas pressure. T−scales linearly with the electron temperature and exceeds the highest values predicted by the theory of dissociative attachment for T−at formation. The possible mechanisms for H−heating are discussed.