The effect of variations in electrolyte and forming conditions on the formation of porous type anodic oxide coatings on aluminum are discussed, with particular reference to the manner in which these variables control oxide formation, pore development, and the thickness of the barrier layer. Formation and solution rate data are applied to show that, during the formation of a porous type coating, conditions at pore bases are vastly different from those existing in the main body of the electrolyte.