Ozone was co-deposited with ICN and, separately, with BrCN in argon matrixes at ca. 14 K, and the FTIR spectra were measured after each photolysis or warming cycle. The experiments were complemented by the analogous studies of ICN and BrCN separately in oxygen matrixes at ca. 14 K. Photolysis of matrixes containing O-3 and ICN led to the formation of bands attributed to an iodoso species, OICN, after only 30 min of irradiation (lambda > 650 nm). UV-vis (lambda > 350 nm) irradiation resulted in the appearance of bands attributed to iodoisocyanate, INCO. Subsequent Pyrex-filtered (lambda > 290 nm) and quartz-filtered (lambda > 240 nm) irradiation increased the yield of INCO and led to the formation of bands tentatively attributed to iodocarbonyl isocyanate, IC(O)NCO, and the acyclic dimer of iodoisocyanate, I2NC(O)NCO. Bands attributed to an iodyl species, O2ICN, were also detected. BrCN is clearly much less reactive than ICN with ozone and required several hours of UV irradiation before any new bands were observed; these are attributed to both bromoisocyanate, BrNCO, and bromocarbonyl isocyanate, BrC(O)NCO. No evidence was obtained for the formation of the halogen nitrile oxides ICNO and BrCNO in argon matrixes. The possible mechanisms of the reactions are discussed.