When low-temperature thin films of either ionic or covalent dinitrogen pentaoxide, N2O5, are exposed to gaseous HCl and water, the only products observed in the solid phase by reflection-absorption infrared spectroscopy (RAIRS) are molecular nitric acid and the oxonium ion. Nitryl chloride, ClNO2, is not detectable. When dinitrogen pentaoxide is co-deposited with hydrogen chloride and water at 85 K and annealed to 140 K, the resultant RAIR spectra indicate that the film is composed of H3O+Cl-, N2O5, HNO3, and D-2h-N2O4 When nitryl chloride is co-deposited with either water or HCl/water mixtures, infrared spectra indicative of solid D-2h-N2O4 are measured, as well as peaks corresponding to nitrate ions and cis-ClONO (chlorine nitrite). Reaction between ClNO2 and its isomer, cis-ClONO, is proposed as an explanation for the formation of dinitrogen tetraoxide in both systems. The proposed reaction mechanism for this hydrolysis is extended to the N2O5/H2O/HCl deposits in order to explain the lack of observable ClNO2 in such thin films.