The dynamics of hydrogen gas bubble evolution at copper microelectrodes in H2SO4 and HCl solutions of various compositions have been studied by means of galvanostatic polarization experiments and simultaneous video taping. As long as the solution contains acid only, gas evolution is dominated by the growth of a single bubble at the electrode at any one time. The transients in H2SO4 solutions exhibit regular sustained relaxation oscillations that can be distinctly related to events occurring at the cathode. The electrode response in HCl solutions is somewhat noisier and shows much larger cathodic polarization and oscillation frequency and amplitude, presumably due to an interaction between Cl- and the copper surface. Additions of Na2SO4 and NaCl to these solutions promote the nucleation and growth of numerous small bubbles at any one time and the diminishing of the oscillations of the electrode potential. The addition of thiourea to 2.0 M H2SO4 does not destroy the single bubble growth characteristic of acid-only solutions, but increases the cathodic overpotential at all organic levels with a maximum occurring at about 0.13 mM thiourea. Oscillation frequency and amplitude, and bubble departure diameter all show a similar dependence on thiourea concentration to that of overpotential.