The electrochemical oxidation of hydrazine was investigated in strongly and weakly pH buffered solutions to reveal the role of buffer capacity in proton-electron transfer redox reactions. In sufficiently buffered solutions, a single voltammetric feature was observed. However, increasing the hydrazine concentration (or, equivalently, moving to an insufficiently buffered solution) gave rise to a second voltammetric feature. These results are rationalized with a conceptually simple model and finite element simulations. We demonstrate that the new voltammetric feature is caused by a large change in the pH at the electrode surface as the reaction proceeds. Importantly, we show that the occurrence of additional voltammetric features are general for proton-electron transfer reactions in insufficiently buffered solutions, and should not be confused with changes in the reaction mechanism.