Photochemical reaction of ethyne.NO2 and propyne.NO2 pairs isolated in solid Ar has been observed at wavelengths as long as 582 nm (HC=CH.NO2) and 615 nm(CH3C=CH.NO2). Continuous wave dye laser radiation was used to excite the reactants, and the chemistry was monitored by FT-infrared spectroscopy. In the case of ethyne + NO2, the sole products were CH2=C=O and NO. The reaction of NO2 with propyne gave methylketene plus a trapped intermediate, CH3C(=NO.)C(=O)H (formyl methyl iminoxy radical). Infrared spectra of this radical are reported for the first time. Product identification was based on D, O-18, and N-15 isotopic substitution. Selective interconversion with tuned visible laser light revealed the presence of four conformers, most probably originating from isomerism about the C-C and the C=N bonds. Photoelimination of NO from the iminoxy radical was observed at wavelengths as long as 625 nm. Propynol and methylketene could be identified among the secondary photolysis products.