The evolution of hydrogen and oxygen from a 0.5 M H2SO4 solution on heavily borondoped diamond electrodes was studied. A very wide potential range without water decomposition, from approximately -1.5 to +2.75 V relative to the standard hydrogen electrode, was observed on high quality diamond. A much smaller range, from -0.7 to 1.7 V, and higher background currents were observed on diamond electrodes with significant sp2 content. The Ce(III)/Ce(IV) redox couple was observable on diamond electrodes, but was highly irreversible.