We report the eleetrochemical generation of a single hydrogen bubble within the cavity of a recessed Pt nanopore electrode. The recessed Pt electrode is a conical pore in glass that contains a micrometer-scale Pt disk (1-10 mu m radius) at the nanopore base and a nanometer-scale orifice (10-100 nm radius) that restricts diffusion of electroactive molecules and dissolved gas between the nanopore cavity and bulk solution. The formation of a H-2 bubble at the Pt disk electrode in voltammetric experiments results from the reduction of H+ in a 0.25 M H2SO4 solution; the liquid-togas phase transformation is indicated in the voltammetric response by a precipitous decrease in the cathodic current due to rapid bubble nucleation and growth within the nanopore cavity. Finite element simulations of the concentration distribution of dissolved H-2 within the nanopore cavity, as a function of the H+ reduction current, indicate that H-2 bubble nucleation at the recessed Pt electrode surface occurs at a critical supersaturation concentration of similar to 0.22 M, in agreement with the value previously obtained at (nonrecessed) Pt disk electrodes (similar to 0.25 M). Because the nanopore orifice limits the diffusion of H-2, out of the nanopore cavity, an anodic peak corresponding to the oxidation of gaseous and dissolved H-2 trapped in the recessed cavity is readily observed on the reverse voltammetric scan. Integration of the charge associated with the H-2 oxidation peak is found to approach that of the H+ reduction peak at high scan rates, confirming the assignment of the anodic peak to H-2 oxidation. Preliminary results for the electrochemical generation of O-2 bubbles from water oxidation at a recessed nanopore electrode are consistent with the electrogeneration of H-2 bubbles.