Photochemistry of OC...HONO complexes in nitrogen matrices has been investigated using lambda>340 nm radiation of a medium pressure mercury arc. Reaction of the OH radicals from HONO photolysis with CO within the nitrogen lattice site has led to formation of trans and cis-HOCO radicals. The HOCO radicals reacted further with NO via two reaction channels yielding trans and cis-HOC(O)NO or HNO+CO2. Comparison of the observed infrared frequencies and isotope shifts (C-13, O-18, and H-2) with the computed frequencies confirmed the assignment of the cis and trans conformers of nitrosoformic acid that have been identified for the first time. In matrices with higher CO concentration the complexes between cis-HOC(O)NO molecule and carbon monoxide were also identified. (C) 2003 American Institute of Physics.