The surface of Si, Ge and GaAs electrodes has been investigated by infrared spectroscopy, which provides in situ information on chemical species at the electrochemical interface. Special attention was paid to adsorbed hydrogen and hydroxyl and to oxide formation. In fluoride medium and at open circuit potential (OCP) the Si surface is completely hydrogenated and is oxidised only upon imposing a large anodic current. Conversely, GaAs is oxidised at OCP, and AsH bonds appear on the surface only in the presence of a cathodic current sufficient to compete with surface oxidation by oxygen dissolved in the electrolyte. Upon potential cycling, the surface of germanium undergoes nearly reversible changes between a hydroxylated and a hydrogenated state, and may be found in either state at OCP, depending upon its history. In situ IR spectroscopy allows one to rationalise the very different behaviours of surface reactivity found for these otherwise rather similar semiconductors and to determine the electrochemical reaction mechanisms.