Low-temperature photolysis of HNIW single crystals was investigated by electron paramagnetic resonance spectroscopy. Crystals of the alpha and beta polymorphs, with either fully or selectively N-15-labeled nitro groups, were photolyzed at 15 K, and various species involving paramagnetic oxides of nitrogen (NO, NO2) were observed after annealing between 15 and 300 K. The orientation of NO trapped in the lattice of beta-HNIW was inferred from the orientation of its g- and N-15-hfs tensors. Regioselectivity of photodecomposition within the two crystal lattices, monitored by the ratio of N-15 and N-14 isotopes in the NO and NO2 radicals, showed that no single reaction pathway produced these products and that concerted three-bond fragmention was not a dominant pathway, Analysis of packing similarities and reaction differences between the alpha and beta polymorphs of HNIW confirmed that the cavities that accommodate water in the a hydrate play an important role in its solid-state reactivity.