Photochemical reaction of ethyne-NO[sub 2] and propyne-NO[sub 2] pairs isolated in solid Ar has been observed at wavelengths as long as 582 nm (HC=V CH-NO[sub 2]) and 615 nm (CH[sub 3]C=V CH-NO[sub 2]). 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 + NO[sub 2], the sole products were CH[sub 2]= C= O and NO. The reaction of NO[sub 2] with propyne gave methylketene plus a trapped intermediate, CH[sub 3]C(= 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, [sup 18]O, and [sup 15]N 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. 45 refs., 7 figs., 5 tabs.