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 the NO2 - moiety in a bidentate planar cyclic structure of C2v 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 Na15NO2 absorbed at 1272.1, 1198.2, and 821.7 cm-1 before irradiation; new lines at 1419.6, 1136.9, and 782.8 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 nm; the numbers grouped in parentheses reflect line splitting.