Infrared absorption spectra of MNO3 (M = Li, Na, and K) in solid Ar were recorded with a FTIR spectrometer. The absorption of KNO3 displayed only singlet lines, at 1460.3, 1293.5, 1031.4, 828.0, 721.4, and 709.4 cm-1, whereas that of LiNO3 and of NaNO3 showed doublets or multiplets due to matrix site splitting. Hence, detailed vibrational analysis of the NO-3 moiety was made for KNO3. From spectra of KN16O3-x18Ox (x = 0-3) and K15NO3, five vibrational modes are unambiguously assigned and a bidentate structure of KNO3 is confirmed. The line at 721.4 cm-1, previously assigned to a B2 mode, is reassigned to the v3(A1) mode. A weak line at 709.4 cm-1, of which 18O-isotopic data are lacking, is tentatively assigned to the v8(B2) mode. A Fermi resonance due to the v1(A1) and a combinational band of v7(B2) + v9(B2) was observed for K(18O18O)N16O at 1465.8 and 1451.7 cm-1. Normal coordinate analysis was performed for the five vibrational modes of all isotopic species. The results are consistent with theoretical predictions using density functional theory (B3-LYP).