In this paper we present the results of electro-optical research of aqueous colloids of nanodiamonds. Application of external electric field to such colloids causes particle orientation, and optical anisotropy caused by such particle orientation can then be easily measured experimentally. These effects allow to determine particle polarizability anisotropy in a wide frequency range. The theory of particle polarizability that takes into account both Maxwell-Wagner and α-dispersion is then used to obtain the magnitude of particle surface conductivity. Since nanodiamond particles are polydisperse and polymorphic, the equations that govern the influence of polydispersity and polymorphism of particles on the observed effects are discussed. To determine the ranges of applicability of particle polarizability theory in electrolyte solutions to diamond particles in aqueous colloids, we studied electro-optical stationary effect in weak electric fields in frequency range 500 Hz–4 MHz at different ionic strengths of the