The kinetics of visible light-induced oxidation of ethyne and propyne by NO2 in solid Ar has been monitored as a function of photolysis wavelength. CW dye and Ar ion laser emission was used for irradiation, and FT-infrared spectroscopy for the measurement of product growth. Determination of the wavelength dependence of quantum efficiencies from the kinetic measurements revealed that the photolysis threshold of HC=CH + NO2 lies approximately 6 kcal mol(-1) higher than that of CH3C=CH + NO2. This and other parameters derived from the product growth measurements, and the formyl methyl iminoxy radical trapped in the case of the CH3C=CH + NO2 reaction are interpreted in terms of a direct O transfer mechanism. The proposed path involves large-amplitude O transfer from photoexcited NO2 to the C=C group to yield a transient ketocarbene. Formation of iminoxy radical is attributed to trapping of the ketocarbene by NO cage coproduct, which is in competition with Wolff rearrangement to yield ketene. From the structure of the iminoxy radical, it is inferred that the photoinduced O transfer from NO2 to the unsymmetrical C=C bond of propyne is completely regioselective, leading exclusively to the carbene transient with oxygen at the terminal carbon. This regiochemical outcome is interpreted in terms of potential energy profiles for central and terminal carbon attack of the triple bond.