Mixtures of ZnO, Sb2O3 and Bi2O3 powders corresponding to the stoichiometric compositions of Zn7Sb2O12 (spinel) and Zn2Bi3Sb3O14 (pyrochlore) were milled in a shaker mill (SPEX) up to 180 minutes. The influence of the mechanical activation of the powders, as a result of the milling, was determined by Differential Thermal Analysis, Thermogravimetric Analysis and X-ray Diffraction. Additionally, in order to compare to commercial recipes the mixture ZnO + 2.7 wt% Sb2O3 + 4.5 wt% Bi2O3 was milled in a shaker mill up to 180 minutes. Varistor devices were fabricated with both milled and unmilled mixtures. The devices were characterized by I-V studies, Scanning Electron Microscopy and X-ray Diffraction. Mechanical activation on the system ZnO-Bi2O3-Sb2O3 produced a chemical reaction and amorphization in the powder mixture. The weight loss due to volatilization of Bi2O3 and Sb2O3 that occurrs in both ZnO-Sb2O3 and ZnO-Sb2O3-Bi2O3 systems at temperatures below 1200°C is reduced by milling the powder mixtures.