Effects of crystal structure on the electrochemistry of boron-doped high temperature-high pressure (HTHP) diamond single crystals grown from a Ni-Fe-C melt are studied. On the {111}, {100}, and {311} faces, the linear and nonlinear electrochemical impedance measurements were performed and the rate of electron transfer for Fe(CN)(6)(3-/4-) was evaluated. Like polycrystalline chemical vapor deposited films, the HTHP electrodes' equivalent circuit includes a constant phase element. The uncompensated acceptor concentration in the semiconductor diamond was determined from Mott-Schottky plots and amplitude-demodulation measurements and was found to vary in the range of 10(18) to 10(21) cm(-3). The difference in the electrochemical behavior of individual crystal faces is primarily attributed to different boron concentrations in the growth sectors associated with the faces.