Sodium nitrite dispersed in solid argon exhibited infrared absorption at 1293.0, 1222:8, and 825.6 cm-1, corresponding, respectively, to symmetric and asymmetric stretching, and bending, modes of cm the NO2- moiety in a bidentate planar cyclic structure of C-2 upsilon symmetry (designated cyclic-NaNO2). After irradiation of the matrix sample with emission from the KrF laser at 248 nm, new lines at 1446.2, 1159.1, and 787.1 cm(-1) were recorded. A sample containing (NaNO2)-N-15 absorbed at 1272.1, 1198.2, and 821.7 cm(-1) before irradiation; new lines at 1419.6, 1136.9, and 782.81 cm(-1) were observed after irradiation. New lines observed after irradiation are assigned to sodium nitrite with a monodentate planar trans structure (designated trans-NaNO2). Ab initio calculations according to density functional theory at Becke3LYP/6-311+G* level predicted line positions, isotopic shifts, and relative IR intensities similar to those observed experimentally for both isomers. Similar results were obtained for cyclic-KNO2 at (1313.4, 1310.8), (1217.3, 1207.9), and (807.2, 809.5) cm(-1) before irradiation, and for trans-KNO2 at (1412.4, 1413.2), (1171.0, 1178.2, 1176.4), and 783.1 cm(-1) after irradiation at 248 nn; the numbers grouped in parentheses reflect link splitting.