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The varistor effect takes place in SnO2-Co3O4-Nb2O5-Cr2O3 ceramics with CaO, SrO or BaO additions prepared by solid-phase sintering and in the same ceramics with CuO, V2O5 or Bi2O3 additions prepared by liquid-phase sintering. This effect is accompanied by a humidity-sensitive effect at the low-field electrical conductivity. The latter is observed when air relative humidity increases from 10 to 93%. The lowest humidity-sensitive effect was found in the liquid-phase sintering samples. For the investigated samples the values of the nonlinearity coefficient 24-50 at high electric fields (E1 = 2.8 − 11 kV cm −1 ) and the humidity sensitivity coefficient 1.8•10 3 -3.2•10 5 at low electric fields were calculated. Such properties of tin oxide based ceramics can be explained within the model of electrical conduction controlled by the grain-boundary potential barriers and the dissociative adsorption of water molecules on the samples surface. This leads to a low decrease of the intergranular potential barrier height in the humid air and an increase in the conductivity in low electric fields. At the same time in the high electric fields the barrier height decreases more intensively with an increase in the electric field and this leads to the varistor effect.