Equations describing formation of a spherical oxide shell according to the Cabrera-Mott mechanism are presented. Two different configurations of metal and oxidizer are considered: oxidation of a spherical metal particle in surrounding oxidizer, and reduction of a spherical oxide inclusion in a metal matrix. Equations for the former configuration were reported earlier but did not explicitly account for volume changes in the growing oxide shell and shrinking central core. For aluminum oxidation, the correction for these volume changes is significant for spherical particles with diameters less than 10 nm. (C) 2011 Elsevier B.V. All rights reserved.