The electrochemical properties of highly doped p-type single crystalline diamond electrodes (100 and 110 oriented) in aqueous electrolytes were investigated. The interfacial capacitance obeys the Mott-Schottky relationship in a considerable potential range and can be assigned to a depletion layer in the diamond. The energetic position of the valence band edge is about 4 and 2 V versus SHE for (100) and (110) oriented diamond respectively. Oxygen and hydrogen evolution occur at large overpotentials (1 V) in agreement with previous results reported for polycrystalline diamond. Interestingly, with reversible redox systems, metal-type redox kinetics around the equilibrium potential are observed. The mechanism of electron exchange between the valence band of diamond and simple redox systems was investigated in detail using electrochemical impedance spectroscopy. A quantitative model is proposed, that assumes that electron exchange is mediated by bandgap states.