Conductivity of composite solid electrolytes (1 − x) CsNO2–xA (A =MgO, Al2O3, SnO2) was systematically investigated. It was found that in all cases heterogeneous doping leads to the increase in the conductivity. Conductivity goes through a maximum as a function of the oxide concentration that is explained by additional contribution of the interface regions. However, in contrast to known composites, at small concentration of oxides the character of the Arrhenius dependences is similar to that of pure cesium nitrite, where intrinsic and extrinsic conductivity regions are observed. At high concentration of the heterogeneous dopant the character of Arrhenius dependences changes and no change in slope of the Arrhenius dependences is observed. According to thermal analysis data, at high concentration of oxide amorphous interface-stabilized phase of CsNO2 forms on the oxide interface. Composite solid electrolytes based on CsNO2 with additives of nanocrystalline γ-alumina and SnO2 have a high ionic conductivities exceeding 10−2 S/cm at 356 °C. Composites containing additives of MgO have lower conductivity despite of high specific surface area.