The time taken for gas bubbles to start forming on an electrode operating in a stagnant aqueous solution is driven by the dissolved gas mass-transfer near the electrode. This is the result of two processes that compete with one another. One adds dissolved gas molecules from the chemical reactions at the electrode surface and the other diffuses these molecules towards the bulk. Using this mechanism, a model is proposed that can predict the dissolved gas supersaturation concentration up to the onset of bubble nucleation at the electrodes (heterogeneous nucleation). Experimental measurement of the bubble onset nucleation time is incorporated within this model to calculate the critical supersaturation concentration for dissolved hydrogen and oxygen gas. The results show a strong link between the applied current density and the supersaturation concentration at the electrode surface. A relation was obtained that predict the onset of bubble nucleation showing an excellent agreement with the measurement. (C) 2014 Elsevier Ltd. All rights reserved.