AbstractThe method of molecular‐modulation spectrometry for studying photochemical reactions has been applied to methyl nitrite photolysis. The infrared absorption of the nitroxyl radical HNO has been observed in the gas phase at 3300 cm−1. Under the present experimental conditions the steady‐state concentration of HNO under steady illumination was 1.1 × 1012particles/cc, and the observed modulation amplitude was 4.5 × 1010particles/cc. At 25°C and 1 atm of nitrogen, the cross section for infrared absorption by HNO at 3300 cm−1is 1.7 × 10−19cm2. The rate constant ratiob/cwas found to be 8.0. From the literature value of the rate constantd, the observed rate constant for the reactionise= (5 ± 1) × 10−