The reaction of hydrogen evolution involving discharge of protons arising from strong and weak acids, namely perchloric, nitric, acetic, ethylenediaminetetra-acetic and dihydrogen phosphoric acids, was investigated in the concentration range 0.1-10 mM by steady state voltammetry at inlaid platinum micro-disc electrodes (of 10-12.5 mu m radius). ?The second-order CE mechanism for the weak acids was rationalised by digital simulation, demonstrating the effects of the various kinetic, thermodynamic and diffusion parameters on the reduction process, In particular, it was found that proton reduction from acetic acid is associated with a completely mobile equilibrium over the whole concentration range studied, but in buffer solutions containing a large excess of the conjugate base the CE process becomes affected kinetically. For the other two weak acids the chemical step controls the kinetics at any concentration employed.