The potential of zero charge of oxide-covered metals (E-pzc) is shown to be linear function of the isoelectric point of the oxide film (pH(pz)c). The linearity is displayed for 14 different metals having n-type semiconductor oxide films and for three metals having p-type semiconductor oxide films. Using experimental values from the literature, the slope of the linear plot of E-pzc vs. pH(pzc) is observed to be -0.142 V for n-type oxides and -0.115 V for p-type oxides. The theoretical slope is -0.120 V. which is derived in this communication. (C) 2009 Published by Elsevier Ltd.