At gas-evolving electrodes, the product of an electrochemical reaction, e.g. H-2 or O-2, does initially not exist as a gas but in dissolved form in the electrolyte liquid. Its appearance in the form of gas bubbles results from subsequent desorption. However, the process of desorption from the liquid phase into the gaseous phase is commonly not complete in the vicinity of the electrode or at all in the interelectrode space. The fraction of the product transferred into the gaseous phase of gas bubbles adhering to the electrode surface is termed the gas-evolution efficiency. This quantity is one of the controlling parameters in estimating the rate of microconvective mass transfer, the industrially most important mechanism. A theoretical investigation shows the impact of the bubble coverage - previously only derived from an analysis of experimental data in combination with an unsubstantiated perception and gives information on further controlling quantities: the mass transfer coefficients of two competing mechanisms. (C) 2010 Elsevier Ltd. All rights reserved.