Under gas evolution, the current voltage characteristics of an electrochemical cell are strongly affected by the presence of growing bubbles. At high current densities, the evolution of the bubbles becomes the rate limiting step of the electrochemical process. The knowledge of the amount of bubbles shadowing the active electrochemical surface of the working electrode has many important practical consequences. This contribution presents a model describing qualitatively the evolution of the bubbles evolving on the working electrode. The theoretical investigations indicate that one of the mechanisms influencing the fraction of active electrode surface is their ability to coalesce or not.