Light-induced charge transport properties of LiNbO//3 crystals containing Mg dopants as well as Fe ions in different valence states were studied to obtain quantitative information on the influence of Mg. Also investigated were LiNbO//3:Fe crystals with different Li/Nb ratios. The experiments indicate that the photovoltaic effect is determined by the Fe**2** plus concentration only and depends weakly on the Li/Nb ratio or Mg doping. The photoconductivity is increased by Mg dopants, thus diminishing the light-induced refractive index changes.