Metal oxide varistors are ceramic semiconducting devices characterized by an extremely nonlinear current-voltage characteristic (MATSUOKA et al.). They are formed by sintering a mixture of ZnO with small additions of Bi203 and other metal oxides at high temperatures according to a conventional ceramic technology. Because of their good voltage limitation properties and their ability to withstand surge currents they fmd wide applications as circuit protectors against voltage transients and power overloads (HAR" et al.). -For all systems (besides the Si0,-free system (l), x = 0) the occurrence of a Si-rich phase -In systems which contain Sb203 an Sb-rich phase (especially enriched with Cr) is present.X-ray phase analyses were carried out according to BATHER et al. ; thereby the detection sensibility of small phase portions could be increased by preferentially etching away the ZnO matrix grains and using a readjusted monochromator for the goniometer step by step technique.(1) Z, Bi, S, Sp (2) Z, Bi, S, Sp is characteristic.In the investigated systems the following phases were detected :