Gaseous nitric oxide was examined from 4000 to 1600 cm−1 at temperatures between 77 and 150°K with a 3.9‐m and a 6‐cm cell. Features found at 1860 and 1788 cm−1 exhibited a quadratic pressure dependence and were assigned to (NO)2. The band shape of these features indicated the presence of a cis‐(NO)2 isomer. The heat of formation of (NO)2 from monomeric NO is ΔH = − 2450 cal/mol. Other bands due to (NO)2 are reported.

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