The kinetics and mechanism of corrosion of GaAs and InP have been investigated quantitatively by means of a combined experimental approach based principally on (i) Tafel plot measurements and (ii) the comparison of n- and p-type electrodes of the same material. In this work (Part I), the motivations and the limit of validity of this approach are discussed in detail along with the description of experimental results. It is shown that an extensive determination of the flat band potential under various electrochemical conditions (called in the following “Tafel plot recording” by analogy with metal substrates) is a very powerful technique, sensitive to a number of parameters of the contact such as (i) the pH of the solution, (ii) the doping density of the electrode, (iii) the type of electrode conduction (n-or p-type) and naturally (iv) the material itself. The present work brings new insight into electrochemical studies of the semiconductor/electrolyte junction and makes a parallel with the electrochemistry with metal electrodes. This part presents only the results. Part II will cover the discussion in relation to the surface electrochemical and chemical properties of GaAs and InP.