Reversible proton conducting solid oxide cells (SOCs) off a highly efficient route to matching supply from intermittent, renewable resources, with power demand by consumers. The cells would store excess electrical energy as chemical fuel during times of peak production, and operate in reverse during times of peak demand. In this study we examine the operation of anode supported proton conducting SOCs in electrolysis mode. The required overpotential for a given current density decreases with increasing humidity at the anode and increasing temperature. All of the V-I curves show distinct curvature. The electrode polarization resistance increases and electrolyte ohmic resistance decreases with increasing current density. This is accompanied by a deviation below the theoretical rate of hydrogen production. We interpret these changes as resulting from deviation away from pure proton conduction in the cell with increasing polarization. (C) 2011 Elsevier B.V. All rights reserved.