The problem of relating electrophoretic mobilities to surface charge densities of particles in suspension has been investigated mainly experimentally. The subject has been approached by comparing the surface potentials at an oil/water interface calculated for adsorbed films of ionic surface active agents, with the zeta potentials of oil droplets in similar surface-active agent solutions. When the bulk viscosity and dielectric constant were used in calculating the zeta potentials, the surface and zeta potentials converged towards low surface charge densities. At higher charge densities the zeta potentials fell well below the surface potentials. The reasons for this discrepancy have been investigated. Misapplication of the Gibbs equation, surface hydrolysis of adsorbed ions, specific interaction of adsorbed ions with counter-ions, surface conductivity, internal circulation in the liquid droplets and double-layer dielectric constants and viscosities were considered. By elimination it has been concluded that only enhanced values of the $\eta$/D ratio in the electrical double layer can account for the experimental observations. Independent evidence for this phenomenon has been briefly examined. The circumstances under which a reliable value of surface charge may be determined from electrophoretic measurements have been discussed.