The dielectric properties of suspensions of spherical polystyrene particles with radii of 34 and 50 nm, in aqueous electrolyte solutions were measured in the frequency range of 10 kHz to 30 GHz using l-IP 4192A impedance and 8753A network analyzers, and double path interferometers. The samples were prepared from the stock suspensions with a high particle concentration (volume fi-action of 0.4), which were used either unchanged or diluted with increasing amounts of water down to a concentration of 0.05. The permittivity and total loss spectra showed three well-defined relaxation regions, which could successfully be interpreted using an existing model [Grosse, C., J. Phys. Chem. 92, 3905 (1988)] (corrected by including a term of counterion diffusion on the particle surface). A model spectral function was fitted to the data using seven free parameters : the static conductivity of the electrolyte solution, three parameters associated with the relaxation of the electrolyte solution, and just three parameters for the description of both the counterion and the Maxwell-Wagner relaxations. For different particle concentrations, the values of the surface conductivity (obtained from the Maxwell-Wagner parameters) remained essentially constant, while the low-frequency relaxation parameters exhibited very good agreement with theoretical predictions in Delgado, A. Ti., Arroyo, F. J., Gonzales-Caballero, F., Shilov, V. N., and Borkovskaya, Yu. B., Colloids Surf. A, in press.