Properties of RbNO3 in (1-x)RbNO3-xAl(2)O(3) nanocomposites were studied by X-ray powder diffraction, electron diffraction, conductivity measurements, infrared and Raman spectroscopy, and differential scanning calorimetry. All the experimental techniques used indicate the presence of an amorphous RbNO3 layer at the RbNO3-Al2O3 interface in addition to a crystalline RbNO3 phase. Its concentration was determined from the phase transition enthalpies. The amorphous layer thickness estimated by means of a simple brick-wall model is 4 nm.