Abstract Conductivity, structure and thermodynamic parameters of CsCl in (1− x)CsCl– xAl 2O 3 composites are investigated. As the heterogeneous dopant, highly-dispersed alumina with specific surface area 210 m 2/g is used. Maximum conductivity 9×10 −4 S/cm at 400°C occurs at x=0.6. As x rises, conductivities of bcc and fcc phases tend to level off and at x>0.6 no conductivity drop due to the phase transition occurs. The phase transition enthalpy decreases with x and at x>0.6 no bcc–fcc phase transition is detected by DSC technique. New peaks attributed to fcc-phase with the lattice parameter a=0.694 nm are observed on X-ray powder diffractograms. At high x conductivity enhancement is likely to be due to the presence of the high-temperature fcc phase of CsCl stabilized on CsCl–Al 2O 3 interfaces.