Visible-light-induced reaction between 1-phenyl-1-propyne and NO2 in low- temperature argon matrixes has been studied by Fourier transform infrared spectroscopy with an aid of density functional theory (DFT) calculation. Infrared bands observed upon 580-nm irradiation are assigned to acetyl phenyl iminoxy radical, which is produced by recombination of acetylphenylmethylene, a ketocarbene intermediate, with a reactive coproduct, NO. Conformations around the OC-CN and C=N axes of acetyl phenyl iminoxy radical are determined by a comparison of the observed and calculated wavenumbers. A small amount of methylphenylketene is produced from ketocarbene by migration of the methyl group. The branching ratio of the migration against the recombination is estimated to be 0.072 +/- 0.006 by an analysis of absorption-growth behavior for the infrared bands of methylphenylketene and acetyl phenyl iminoxy radical.