In facing the design of new processing strategies for ZnO based ceramic varistors, a precise control of its microstructural development during sintering is demanded. Addition of dopants to zinc oxide results in the formation of secondary phases that to a large extent determine the macroscopic electrical properties of the ceramic. In a varistor system based on ZnO with small additions of Bi2O3 and Sb2O3 these three oxides govern the reactions at high temperature that give place to the secondary phases. These reactions become then the head point from which the functional microstructure is configured. In this way the present work deals with the thermal evolution of the ZnO-Bi2O3-Sb2O3 system in the region of interest for varistors, revealing the existence of two simultaneous reactions paths during sintering these ceramics. (c) 2006 Springer Science + Business Media, Inc.