Hydrogen adsorption and desorption at polycrystalline platinum electrodes in a neutral Na2SO4 solution were studied with a scanning electrochemical microscope. Experiments were carried out with the tip-substrate voltammetry mode, where the faradaic current flowing to the tip is recorded while cycling the potential of the substrate, and with the tip-substrate chronoamperometry mode, where the tip faradaic current is recorded as a function of time following the application of a potential step to the substrate. The tip current was made pH sensitive by holding the tip potential in a region where a pH-dependent reaction occurs. Proton reduction was used to monitor pH decrease, whereas platinum oxide formation was selected to detect pH increase. The results showed that a transient pH decrease as high as 2.3 pH units exists during hydrogen desorption and that a great pH increase occurs during hydrogen adsorption. The mechanisms of hydrogen adsorption and desorption were analysed by comparing tip current vs. substrate potential curves, which reflect the exchange of H+ between the adsorbed layer and the solution, with substrate current vs. substrate potential curves, which reflect the exchange of electrons between the adsorbed layer and the electrode. New conclusions have been drawn.