The characteristic difference in the diamagnetic fractions of hydrogen in doped and undoped amorphous hydrogenated silicon (a-Si:H), as obtained through MuSR modelling, is investigated by first reviewing recent calculations of the electronic energies of interstitial hydrogen. This results firstly in the identification of a charge transfer process between dopant atoms and hydrogen interstitials that provides a new mechanism for dopant passivation. It is concluded, however, that the increase in the diamagnetic fraction with temperature in doped a-Si:H can only be explained by the diffusion of hydrogen interstitials to charged dangling bond sites. These two interactions, of interstitial hydrogen and dopants on the one hand, and hydrogen and dangling bond defects on the other, are then used to propose a hydrogen pathway between dopants and dangling bond defects that explains why the process of doping amorphous silicon is always accompanied by the formation of charged dangling bonds. (C) 2003 Kluwer Academic Publishers.