At all gas-evolving electrodes, the fraction of the electrode surface area covered by adhering bubbles, the so-called bubble coverage, is of substantial effect on the cell operation. A simple empirical relationship is presented to correlate the bubble coverage with the current density. For better understanding the processes, a general relationship is derived showing that numerous further parameters are involved, particularly temperature, pressure and diffusion coefficient, further some parameters which are interrelated with the current density such as supersaturation, gas evolution efficiency and bubble break-off diameter. Experimental investigation is applied to the variation of the break-off diameter and the time-variant change of the bubble coverage. (C) 2004 Elsevier Ltd. All rights reserved.