Low-temperature (3.5<T<77 °K) measurements of the dielectric constant and dissipation of ZnO ceramic varistors are presented for frequencies f in the range 30<f<105 Hz. A loss peak and dispersion in ε observed at 4 °K are identified as resulting from the two-phase structure of the ZnO varistor (Maxwell-Wagner model). We find the zinc oxide grain resistivity ρZnO∼106 Ω cm in a typical commercial varistor at 4.2 °K. The physical origin of a second loss peak observed in measurements around room temperature is discussed. We show the latter dispersion is not interpretable using a Maxwell-Wagner model for an inhomogeneous conductor.