Abstract: Recently we observe the growing interest in the field of nitroso oxides being isomers of nitrocompounds. They are strong oxidizers, can eliminate singlet oxygen and generate atomic oxygen. On the base of our quantum chemical calculations we have assumed that there exists a possibility of in situ formation of nitroso oxide-like fragments in the course of photochemical oxidation reactions of nitrosocompounds, olefins and SO2 by nitrocompounds. We used DFT/6-31G(d) and DFT/6-311+G(d) methods from the Gaussian-03 program package. Identification of the nitroso oxide moieties we have performed on the comparison of the calculated geometries and spectra of the reagents in the transition states with corresponding values known from experiment and/or previously made quantum chemistry calculations.