EQCM responses for Pt electrodes prepared by electrodeposition show the development of large frequency changes in the region of H-adsorption. These changes are not present on electrodes when they are first prepared, but are induced by extended cycling which is shown to result in loss of material from the electrode and in a decrease in the roughness of the electrode surface. They develop to such an extent that, in acid solutions, the frequency change over the H-adsorption region exceeds that seen over the oxide formation region. The large frequency changes seen as the H upd coverage develops disappear gradually as the pH of the contacting electrolyte increases and are associated with the presence of weakly adsorbed hydrogen at the electrode surface. The effects are suggested to be the result of an interaction between weakly adsorbed hydrogen and the contacting water leading to a localised increase in structure at the electrode surface. As pH increases, the increasingly negative charge carried by the Pt surface when adsorbed H is present acts to decrease and eventually remove the effect, presumably because of a less favourable interaction between the charged electrode and the aqueous solvent. The coupling between surface and solution also has a strong influence on the frequency response in the H-adsorption region, with extensive potential cycling and the consequent decrease in roughness leading to an apparent increase in coupling.