The microgravimetric characteristics of electrodeposited nickel and composite Ni/Co hydrous oxide films on Au electrodes in KOH electrolytes were examined in situ with a combination of quartz crystal microbalance (QCM) and voltammetric techniques. In the case of freshly prepared alpha-Ni(OH)(2) hydrous oxide films, denoted as alpha-Ni(OH)(2)(hyd), the mass increased during oxidation and then decreased upon subsequent reduction. As the redox cycling was continued, however, this behavior gradually reversed, i.e., the mass decreased upon oxidation and then increased following further reduction. This unique effect could be correlated with a shift in the position of the oxidation peak maximum (E(p)(ox)) in the voltammogram toward more positive potentials, which has been ascribed to the transformation of alpha-Ni(OH)(2)(hyd) into beta-Ni(OH)(2)(hyd). Based on these findings, it can be concluded that the mechanism of ionic charge transport in these two types of lattices is markedly different. In contrast, composite Ni/Co (9:1) hydrous oxide films displayed qualitatively the same behavior as pure freshly prepared Ni(OH)(2)(hyd) in the same alkaline media regardless of the extent of cycling. This observation provides evidence that the incorporation of cobalt into Ni(OH)(2)(hyd) modifies the structural properties of the lattice during the oxidation/reduction process.