The electrochemistry of gold in 50 mM aqueous sulfuric acid solutions under conditions typical of neural stimulation (cathodic-first current pulsing at 50 Hz) was studied in oxygen-free and oxygenated solutions. As the electrode was pulsed from +0.75 V vs. a reference hydrogen electrode using a 100 mu s, -500 mA/cm(2) pulse in oxygen-free solutions, (i) the initial 10 mu C/cm(2) of injected charge was stored in the double layer, (ii) the next 20-30 mu C/cm(2) was accommodated by double-layer charging plus an undetermined process consuming about 1.8 mu C/cm(2), and (iii) when the electrode potential reached approximately -0.52 V, water reduction consumed all further charge. In oxygen-saturated solution, the unrecoverable charge (associated with irreversible reactions) was not significantly different from the degerated situation throughout the pulse period, but the electrode potential was less negative, consistent with oxygen reduction. Oxygen reduction appears reversible during a 100 mu s pulse, provided the electrode is clamped back to +0.75 V immediately after the pulse. If an open-circuit period was introduced between the current pulse and application of the reversal potential, unrecoverable charge accumulated more rapidly in oxygen-saturated solution, suggesting that processes occurring during the interpulse interval of neural stimulation may produce potentially damaging reaction products. (c) 2005 The Electrochemical Society. All rights reserved.