The ζ potential of quartz and sapphire plates in contact with aqueous solutions was measured by means of the plane interface technique, which involves establishing the liquid flow velocity-depth profile using reference (polystyrene latex) particles. The electroosmotic velocity obtained by extrapolation of the velocity profile to the cell wall permits calculation of the ζ potential of the cell wall-solution interface. The plane interface technique was also used to obtain the ζ potential at aqueous sodium dodecyl sulfate (SDS) solution surfaces with an open cell in which the upper boundary consists of an air-solution interface and the lower boundary, a quartz-solution interface. It was essential to eliminate the meniscus in the open cell to obtain normal velocity profiles. The ζ potential of the aqueous SDS solution surface paralleled the Stern potential, calculated from the surface tension-SDS concentration data by use of Gibbs adsorption equation and Gouy-Chapman theory, though the former was larger in magnitude than the latter.