A laser Doppler apparatus equipped with narrowly gapped platinized platinum electrodes was used to study the electric properties of bitumen-in-water and Moutray conventional crude-in-water emulsions over a wide range of NaCl concentrations and pH. It was found that a dilute buffer solution had to be used to prevent pH drift from occurring on the electrode surfaces. Analysis of the measured mobility using the Ionizable Surface-Group model (K. Takamura and R. S. Chow, Colloids and Surfaces 15, 35 (1985)) revealed that the negative charges of both emulsions were due to the dissociation of carboxylic acids, though Moutray crude oil had only a quarter of the acid groups that existed on the bitumen surfaces. Experiments were also conducted in the presence of Ca 2+ and Mg 2+. The measured mobilities for both emulsions were again adequately explained by the model when the ion-binding of the cations with the acid groups was included. Several mobilities measured under the presence of the strong electrophoretic relaxation (10 −4 M NaCl and CaCl 2) exceeded theoretical maximum values predicted by O'Brien and White's computer solution (R. W. O'Brien and L. J. White, J. Chem. Soc., Faraday Trans. 2 74, 1607 (1978)), thus suggesting that their theory might overestimate the relaxation effect.