Infrared spectra of 20 kinds of monosubstituted quinolines, quinoline 1-oxides, and isoquinoline 2-oxides, and 18 kinds of monosubstituted quinoline 1-oxides were determined. By comparison with the spectra of corresponding quinolines, two strong absorptions in the regions of 1280∼1340 cm^-1 and 1210∼1260 cm^-1 of the N-oxide compounds were assigned to the characteristic N-O absorptions. By examining the solvation effect of methanol and water, however, it was clarified that the N-O stretching vibration contributes to two or more absorptions in the region of 1150 and 1350 cm^-1 due to complex coupling with the vibrations originating in the quinoline ring and ring hydrogens, and there was not found any correlation between these frequencies and the structure. Ring CH out-of-plane bending frequencies were assigned and interpreted by considering the number of adjacent free hydrogens on heterocyclic and carbocyclic rings, and it was found that all the frequencies of monosubstituted quinoline 1-oxides, except the band near 770 cm^-1 in 4-substituted derivatives, show an appreciable shift to a lower frequency than those of the corresponding quinolines.