We measured the dielectric properties of suspensions of colloidal particles over a broad frequency range from 5 Hz to 13 MHz. The dielectric response is characterized by two distinct dispersions alpha and beta that occur at low kilohertz and meghhertz frequencies, respectively. The alpha-response is identical with the counterion relaxation previously observed. The beta-response is a Maxwell-Wagner effect resulting from the interaction of the suspended particles surrounded by a surface admittance element with the medium. The particle and surface admittances display relaxation only in the alpha-range. The beta-effect is strong for particles suspended in an electrolyte of low salt concentration. It is weak for high concentrations and does not significantly contribute to the dielectric response of biological cell suspensions and tissues. The beta-response permits calculation of the surface admittance at high frequencies. The results show that surface conductance and capacitance are independent of frequency above the alpha-range.