Low-temperature photolysis of alpha-HNIW (2,4,6,8,10,12-hexanitrohexaazatetracyclo[5.5.0(5,9).0(3,11)]dodecane) generated three (NO2)-N-15/(NO2)-N-15 radical pairs that were studied by single-crystal electron paramagnetic resonance spectroscopy at 80 K. In two of the pairs, A and B, the two NO2 molecules occupy solvent cavities that are related by unit translation along the a-axis of the alpha-HNIW unit cell (radical pair separations of 9.26 and 9.67 Angstrom). In the third pair, C, they occupy cavities related by translation along the b-axis (similar to 13 Angstrom). Pair formation was highly specific since no pairs were trapped in cavities related by the b-glide (8.14 Angstrom). All three observed pairs survived annealing to 215 K, but from 220 to 294 K they decayed, one by one, without interconverting or separating to isolated radicals. Positive and negative exchange couplings as large as 0.004 cm(-1) were estimated through approximate simulation of the position and intensity of transitions involving the nuclear states with hyperfine-induced S-0-T-0 mixing. Pairs B and C seem each to exist in two forms with a single NO2/NO2 structure but different values of J, due perhaps to different arrangement of nonbonded molecules between the radicals. Implications for the mechanism of solid-state HNIW decomposition and the limited mobility of small molecules in lightly damaged crystals are discussed.