Surface mass titrations of a self-assembled monolayer (SAM) of 15-mercaptohexadecanoic acid on a gold/quartz crystal microbalance electrode have been performed at various electrode potentials. The apparent pK(a) of the monolayer, obtained from the pH value at half the total frequency change, shifted in a negative direction from 6.4 at -200 mV to 4.3 at -700 mV vs Ag/AgCl. When potentials more positive than 600 mV were applied, the pK(a) has a tendency to shift in the positive direction. These results demonstrate that the surface acid/base properties of the SAMs can be controlled by the electrode potential.