The surface structure of Cu(100) electrodes in perchloric acid solutions of pH 1 to 3 was studied in the potential range of hydrogen evolution by video-rate scanning tunneling microscopy, focusing on the recently reported hydrogen-induced surface reconstruction [H. Matsushima et al., J. Am. Chem. Soc. 2009, 131, 10362]. Potential-dependent measurements reveal a two step formation process: at potentials close to the onset of hydrogen evolution a p(1×8) phase emerges, where Cu surface atoms in stripe-like structures are laterally and vertically displaced; at ≈30 mV more negative potentials a transition to a c(p×8) structure with an expanded Cu surface lattice occurs. Correlation of these observations with electrochemical data and studies on hydrogen interactions with Cu(100) surfaces under vacuum conditions support that these phases are induced by hydrogen in subsurface sites, pointing towards a high hydrogen coverage on this electrode surface under reaction conditions.