Using DC and AC electrolysis of water, experimental current density voltage curves were determined using graphite electrodes in an electrolyte of 5 mol of NaOH per liter. It was found that certain aspects of the DC curves could not be accounted for, using a model based on modern electrochemical theory. However, the experimental data could be explained using a revised model which incorporated the adsorption of hydrogen and oxygen by the graphite electrodes. The AC curves had to be explained via a more complex model based on current flowing in the outside circuit. During AC electrolysis, little or no gas was formed via current through the cell. If the electrodes are of a different area, hydrogen and oxygen are formed on the small-area electrode only at a relatively low potential; at a higher potential, both gases are formed at both electrodes. According to the model, the current does not cross the electrolysis cell either above or below the decomposition potential, but flows through the low-impedance outside circuit. Using these models, the experimental features of both DC and AC electrolysis of water can be explained. (C) 2010 Elsevier Ltd. All rights reserved.