Herein we report on the reactivity of NH produced from photolytically induced decomposition of HN3 in Ar matrices at 12 K. This reactivity was experimentally probed in matrix-isolation experiments and theoretically by detailed quantum chemical calculations. The mechanisms for reactions of HN with N-2 and CO were examined. The results of the experiments show that triplet NH((3)Sigma) is formed as a detectable product of the photoinduced HN3 decomposition, indicating that the reaction of HN in its triplet electronic state with N-2 to give HN3 ((1)A(')) is opposed to a significant barrier. Using quantum chemical methods [coupled-cluster single double triple, complete active space self-consistent field, second-order Moller-Plesset] the barrier of this spin-forbidden reaction was estimated to be about 104-130 kJ mol(-1) (depending on the level of theory), in good agreement with the results of earlier experimental and theoretical studies. On the other hand, our experiments show that HN((3)Sigma) reacts under matrix conditions with CO to give HNCO ((1)A(')). The calculations show that the barrier for this reaction indeed is significantly lower (similar to35 kJ mol-1) than the one for the reaction of HN((3)Sigma) with N-2. Triplet-singlet conversion in both systems is achieved either through spin-orbit coupling or interaction with the "phonon-bath" of the Ar matrix environment.