The hydrogen concentration cH and its spatial distribution in a series of glow discharge a-Si specimens, doped in the gas phase or by ion implantation, have been investigated by the 15N nuclear reaction. The results show : (i) cH in the bulk of gas phase doped material depends on the gaseous doping ratios ; (ii) cH in the surface generally deviates from cH in the bulk within a depth of about 500 Å ; (iii) in specimens doped by ion implantation εf can be moved throughout the mobility gap without producing changes in cH