A simple photometric method for determining the electrophoretic mobility of nano- and microparticles in reverse micelles and in solvents with a low dielectric permittivity (2-5) has been developed. The method is based on the use of a thermostatically controlled diaphragm-based optical cell (length 2 cm) with three vertical plane-parallel electrodes (2 x 3 cm; interelectrode gap, 0.3 cm) placed into a standard photocolorimeter. When an electrostatic field (100-600 V) is applied, the particles begin to move away from the electrode of the same polarity. The path traveled by the particles for a given time (2-30 s) is calculated from the change in the optical density of the solution in the near-electrode zone. The electrophoretic potential of nanoparticles in the model systems, calculated from the values of electrophoretic mobility by Huckel-Onsager theory, varied from 70 (Ag nanoparticles in AOT micelles in decane) to -73 mV (aggregated SiO(2) nanoparticles in a decane-chloroform mixture). Calculations by the classical Deryaguin-Landau-Verwey-Overbeek (DLVO) theory determined the contribution of the electrostatic interaction to the stability of the studied systems. We have shown that the surface charge of nanoparticles permits: (1) an electrophoretic concentration of the charged nanoparticles (Ag) with an enrichment factor of up to 10(4), (2) the separation of nanoparticles with zero (C(60)) and a high (Ag) electrokinetic potentials, and (3) the formation of electrostatically bound aggregates (Ag-SiO(2)) through the heterocoagulation of oppositely charged particles.