The composite solid electrolytes (\mathrm{NaCl}-x) mole (% \mathrm{Al}{2} \mathrm{O}{3}(0 \leq x \leq) 50 ) have been prepared by conventional as well as solution casting methods and have been investigated by means of complex imped. ance analysis, X-ray diffraction (XRD), differential thermal analysis (DTA), and scanning electron microscopy (SEM). The composites prepared by conventional method exhibit about an order of magnitude lower conductivity than those prepared by solution casting technique. The XRD and DTA results show that no phases other than (\mathrm{NaCl}) and (\mathrm{Al}{2} \mathrm{O}{3}) are present. SEM results show that (\mathrm{NaCl}) grains are interspersed with (\mathrm{Al}{2} \mathrm{O}{3}) particles. The conductivity of dispersed systems depends on the processing and the particle size and concentration of (\mathrm{Al}{2} \mathrm{O}{3}). The mechanism of excess conductivity is proposed to be the generation of excess sodium ion vacancies in the host matrix at the matrix-particle interface. Macroscopically, the results are satisfactorily explained on the basis of random resistor network model by assuming the formation of high conducting space charge layer along the matrix-particle interface. 1994 Academic Press, Inc.