Two conformers (cis and trans) of potassium peroxynitrite (KOONO) were produced in an argon matrix containing potassium nitrate (KNO3) at 13 K by means of in situ photolysis with an ArF excimer laser at 193 nm. Photoconversion among cis- and trans-KOONO, and KNO3, was achieved on irradiation of the matrix with a laser at varied wavelengths. With the aid of the relative intensities of IR absorption lines observed for each species at each stage of photolysis, the UV absorption spectra of cis- and trans-KOONO were determined. The absorption maxima, near 325 and 375 nm for cis- and trans-KOONO, respectively, agree with theoretical calculations by Krauss. The photolytic behavior of both conformers at varied wavelengths can be understood in relation to the observed UV absorptions.