The general conditions for doping are considered. Doping by hydrogen is then considered, and whether a single rule separates those oxides in which hydrogen forms only deep states and those in which it acts as a donor. Hydrogen is calculated to act as a shallow donor in oxides ZrO2, HfO2, SnO2, La2O3, Y2O3, TiO2, SrTiO3 and LaAlO3, but it is deep in the oxides SiO2, Al2O3, ZrSiO4, HfSiO4, and SrZrO3. (C) 2003 Elsevier B.V. All rights reserved.