Equilibrium defect concentrations calculated by numerical modeling based on a point defect model showed that protons and electrons were predominant defects under reducing and high pH(2)O conditions. Hydrogen permeation flux for 1 mm SrCe0.95Y0.05O3-d membranes was derived on the basis of chemical diffusion theory as functions of pO(2) and pH(2)O. Hydrogen permeation flux keeps increasing with increasing water vapor pressure gradient with a reference to 10(-6) atm of pH(2)O.