The microscopic mechanism of hydrogen gas evolution on GaAs(100) cathodes is elucidated by studying the reaction intermediate, adsorbed hydrogen, using in situ infrared spectroscopy in realtime. The spectra show that hydrogen adsorbs cathodically at arsenic sites only and replaces As-OH groups present in the anodic range. Absolute submonolayer coverages by adsorbed hydrogen and variations in surface dipole potential due to hydrogen adsorption are both determined quantitatively. Molecular hydrogen is formed upon proton reduction at As-H sites. The microscopic reaction mechanism at GaAs is found to be pi-I-independent; the reason is that rate-limiting electron transfer is not to solution species but to As-H surface sites. A simple mechanistic model accounts for the infrared spectroscopic results as well as for the electrochemistry, explaining both the approximately Linear potential dependence of hydrogen surface coverage and the hysteresis loop in cyclic voltammograms.