The kinetics of photooxidation of para-methoxyphenyl azide 1 was studied by flash photolysis with spectrophotometric detection of the absorption of active intermediates in an aerated acetonitrile solution at 295 K. The holistic set of experimental data including the consumption of cis-trans isomers of para-methoxyphenyl nitroso oxide 2 and the accumulation of photooxidation products (2Z,4E)-4-methoxy-6-oxo-hexa-2,4-diene-nitrile oxide 3 and bis-p-methoxy-azobenzene 4 monitored via the changes in the optical density of the solution in the wavelength range of 300-500 nm was treated to obtain the most complete information about the system under study. Flash photolysis of 1 results in the formation of the corresponding triplet nitrene, which either recombines to azobenzene 4 with a rate constant 2k4 = (8.7 ± 1.0) × 108 L/(mol s), or adds molecular oxygen to produce cis-trans isomers of nitroso oxide 2 with a total rate constant k5 = (1.0 ± 0.1) × 106 L/(mol s). The latter reaction is the main channel for nitrene consumption: the yield of 2 per consumed nitrene was found to be 90% higher at the cis/trans isomers ratio of 0.94 ± 0.04. The trans-isomer of 2 is chemically inert under the experimental conditions and is consumed in the trans-cis conformational transformation with the rate constant k6 = 0.45 c-1. The cis-isomer is thermodynamically more stable, k-6 = 0.16 c-1, but it is consumed irreversibly in the ortho-cyclization reaction with the rate constant k7 = 12.5 c-1. The final product, nitrile oxide 3, has intense absorption in the near UV region, λmax = 300 nm and εmax = 2 × 104 L/(mol·cm). This made it possible to determine the spectral characteristics of 2 isomers using nonlinear regression analysis. It was found λmax = 421 nm, εmax = (1.23 ± 0.07) × 104 L/(mol·cm), and δ = 1640 ± 10 cm-1 (half width at half maximum) for cis-2 and λmax = 462 nm, εmax = (1.57 ± 0.03) × 104 L/(mol·cm), and δ = 2150 ± 10 cm-1 for trans-2. The obtained spectral parameters are in good agreement with the results of optical spectra modeling for 2 isomers performed in the CIS(D,Full)/aug-cc-PVTZ//CASSCF(14,13)/ma-def2-TZVPP approximation: the calc-exp deviation for the absorption maximum was 0.05 eV (cis) and 0.12 eV (trans), the oscillator strengths were calculated to be 0.59 and 0.65, respectively.