The main primary photochemical process for alkyl nitrites is fission to nitric oxide and alkoxy radicals, a proportion of which may be sufficiently excited to decompose spontaneously. The photolysis of t-butyl, of isopropyl and of ethyl nitrite at 254 nm, in the gas phase, each yields nitrosomethane and an aldehyde or ketone. The overall reaction, in presence of nitric oxide, corresponds to: ▪ where R′ and R″ are either H or CH 3. The dependence of the quantum yield of nitrosomethane and of other products on the pressure of alkyl nitrite, of nitric oxide and of other added gases was investigated to seek confirmation of the excited alkoxy mechanism, to determine primary yield and to obtain information about the deactivation of excited radicals. The results for t-butyl nitrite fully confirm the excited alkoxy mechanism. A value of 0.87 is derived for the quantum yield of t-butoxy radicals sufficiently excited to undergo decomposition, in excellent agreement with a previous estimate. The results for ethyl nitrite also support the excited alkoxy mechanism, with the quantum yield of excited ethoxy radicals estimated to be 0.45. The results for isopropyl nitrite differ in several respects from those for the other two systems. A contribution from a primary process involving an intramolecular rearrangement to nitrosomethane and acetaldehyde cannot be excluded, although an alternative explanation in terms of an excited alkoxy mechanism is suggested.