Conductivity of composite solid electrolytes CsNO2–A (A = MgO, Al2O3, SiO2) were systematically investigated. It was found that in all cases heterogeneous doping leads to the increase in the conductivity. Composite solid electrolyte based on CsNO2 with additives of nanocrystalline γ-alumina has the highest ionic conductivities exceeding 3.6 ∙ 10−2 S/cm at 629 K. Composites with MgO have lower conductivity and contain nitrate groups formed during synthesis. In general, composites containing basic oxides exhibit stronger conductivity enhancement on heterogeneous doping. Composites containing additives of SiO2 have lower conductivity despite of high specific surface area that is probably caused by its acidic nature. The results of the conductivity studies are in a qualitative agreement with the 133Cs NMR data: the character of the variation of the NMR spectra is similar for CsNO2 – A (A = Al2O3, MgO) composites and differ from those for CsNO2 – SiO2 composite.