We present a four-electrode dielectric spectrometer, particularly developed for measuring relaxation characteristics in the impedance of colloidal dispersions. The instrument is optimized for covering the frequency range from approximately 500 Hz to 500 kHz. The set-up consists of a cell connected to a signal-Processing unit and a commercial frequency response analyser. The four-electrode design avoids effects of electrode polarization. The electronic circuitry is optimized for measuring small differences between the impedances of samples and blanks, each with extremely small phase angle values (of the order of 0.0001 rad).