FTIR spectra of solid ClNO2 in interaction or not with amorphous water ice between I I and 200 K were analyzed, and the photochemistry of these solids was investigated. When ClNO2 is deposited on an ice surface at 30 K or trapped in the water lattice, no ionization or reaction with water is observed even after annealing from 30 to 180 K. Photolysis of thin ClNO2 films at 266 nm produced dinitrogen tetra-oxide as primary product. When ClNO2 is trapped in ice, exposure to ultraviolet light quantitatively converted ClNO2 to nitric acid hydrates.