Explicit equations correlating oxygen nonstoichiometry to oxygen partial pressure and temperature are important for applications of perovskite-type ceramics as membranes, adsorbents and catalysts in various chemical reaction and separation processes. A semi-empirical equation for oxygen nonstoichiometry on perovskite-type ceramics is reported in this paper. Though derived from the results of a point defect model on a perovskite-type ceramic material, La0.1Sr0.9Co0.5Fe0.5O3-delta, this equation describes very well the experimentally measured oxygen nonstoichiometry data for two perovskite-type ceramics measured in this work and three perovskite-type ceramics reported in the literature. The major advantage of this semi-empirical equation lies in its simplicity, explicitness and accuracy. This equation is coupled with oxygen permeation equation to predict oxygen permeation current density through two perovskite-type ceramic membranes. The predicted data agree very well with the results reported in the literature using a complex defect reaction model.