Growth of oxide scales on metals is often accompanied by the development of porosity in the oxide or at the metal‐oxide interface. These irregularities should affect the kinetics of the reaction and, if ignored, may lead to incorrect conclusions about the mechanism of the reaction. A number of simple models in which pores are assumed to grow in the oxide or at the interface are investigated, and it is shown that a sequence of ranges governed by different kinetics might be found in several cases. Recent experiments which seem to contradict Wagner's theory of scale growth are discussed and an explanation in terms of pore growth is offered.