A double-detector system with which mobilities can be determined by capillary zone electrophoresis is described. Ionic species are detected by two detectors, mounted at a fixed distance from each other. In zone electrophoresis the velocity of an ionic species is proportional to its mobility. The time needed for an ionic species to pass both detectors is reversely proportional to that mobility, provided that the electric field strength is constant. From the ratio of the times required by a sample ionic species and a standard ionic species (with a known mobility) to pass from the first to the second detector, the mobility of the sample ionic species can be calculated. During the analyses parameters such as temperature, pH and ionic strength are constant, which simplifies the calculations substantially. From experiments with some standard ionic species, mobilities have been obtained comparable to those in the literature with a relative standard deviation of about 1%.