We point out that a correct mechanism for the reactivation of shallow impurities in hydrogenated crystalline silicon has to interpret the facts that the activation energy of the reactivation is about 1.4eV which does not depend on the type of dopants and dopant species, the reactivation rate can be enhanced by minority carrier injection or light illumination. The current mechanism is intuitive and is unjustified by the experimental facts. A microscopic model for the reactivation involving hydrogen-related complexes which do not contain any other impurities is proposed. Reactivation of shallow impurities occurs when this complex dissociates creating interstitial hydrogen atoms which diffuse rapidly to hydrogen-impurity complexes and break hydrogen-passivating bonds, and thus a hydrogen molecule is formed. , A possible candidate of this hydrogen-related complex involving two-hydrogen-atom complexes is also suggested.