Electrophoretic mobility measurements on highly charged sulfonate/styrene latices have been performed to test the validity of the relaxation term associated with the theory of O’Brien and White (J. Chem. Sec. Faraday Trans. 2 1978, 74, 1607). This theory links the electrophoretic mobility of a particle to the potential of the plane of shear. Good correlation with the theoretically predicted maximum mobilities and experimental observations has been observed.