The effect of Co, Cr and Nb on the electrical properties of the grain boundaries of SnO2-based varistors was investigated. The powders were prepared by the method of evaporation and decomposition of solutions and suspensions. Varistor samples were obtained by uniaxial pressing followed by sintering at 1300 degrees C for 1h. The electrical properties of the grain-boundary region, such as resistance (R) and capacitance (C), were determined using ac impedance spectroscopy in the 27-330 degrees C temperature interval. Activation energies for conduction (E-A) were calculated from the Arrhenius equation. The non-linear coefficients (alpha) and the breakdown electric fields (E-b) of the samples were determined from the current-voltage characteristics. The potential barrier height (Phi(b)) was calculated using the Schottky-type conducting model. After a comparison of the characteristic parameters for different varistor compositions it was found that the Cr/Nb ratio has a crucial influence on the grain-boundary properties in SnO2 varistors.