In situ observation of the hydrogenation processes of Si(111)-7 X 7 was carried out using an ultrahigh vacuum electron spin resonance (UHV ESR) system. The termination reaction of the surface dangling bond with hydrogen, including the isolating effect of the dangling bond, was observed. The reactivity of hydrogen molecules with the Si(111)-7 X 7 surface is much less than that of oxygen. The g-value anisotropy showed that the signal originated from adatom dangling bonds of the Si(111)-7 X 7 structure. Thermal annealing at a temperature higher than 400 degreesC promoted hydrogen desorption. In both hydrogenation and annealing processes the maximum of the effective spin density observed is approximately 3 X 10(13) cm(-2).