Vacuum ultraviolet (VUV) photolysis of an HCl/O-2 mixture has been carried out to identify HOOCl by the Ne matrix-isolation infrared spectroscopy with the aid of quantum chemical calculations. Newly-observed IR bands at 823, 1363 and 3542 cm (1) are assigned to those of HOOCl on the basis of the isotope shifts with DOOCl and the CCSD(T)/aug-cc-pVDZ calculation. The observed dependence of band intensities on the time of VUV photolysis indicates that HOOCl and HOClO are the photoproducts formed in an early stage. In addition, HOOCl is found to decompose to form HCl in the UV photolysis (lambda >= 365 nm), which consists with the S-1-S-0 energy separation estimated by the TDDFT method. (c) 2009 Elsevier B.V. All rights reserved.