Abstract Infrared (IR) spectra do not show changes for hydrated or deuterated RbNO 3 samples, where no hydrogen (deuterated) bondings are formed. However, in the presence of the second Al 2O 3 phase, a splitting band at the high frequency, 1425 cm −1 and a large wavenumber shift from 1380 to 1135 cm −1 are observed for the ν 3 mode in the hydrated and deuterated RbNO 3-Al 2O 3 samples. These changes results from the coordination of HNO 3 and the isotopic effect of HNO 3 and DNO 3 bondings. This isotopic effect is very similar to that of the H-O and D-O bondings. From the IR study, it is suggested that the proton conduction in these composite materials is dominated by an interfacial conduction mechanism.