The photolysis of HN/sub 3/ in the presence of dimethylacetylene in 12 K matrices gives reaction products in Xe different from those formed in Ar. In Xe the reaction results in N-methyl-1-amino-1-propyne as the major product, whereas in Ar the primary product is 3,3-dimethylketenimine. In Xe, 3,3-dimethylketenimine is produced in low yield, and in both matrices another product, probably 2,3-dimethyl-1H-azirine, is formed as a minor product. It is postulated that, in Ar matrix, photolysis of HN/sub 3/ gives NH(/sup 1/..delta..) which reacts on a singlet reaction surface to give, primarily, the ketenimine. In Xe matrix, however, the external heavy-atom perturbation promotes intersystem crossing so that reaction occurs on a triplet reaction surface, leading to a different product distribution that favors the propyne. 13 references, 9 figures, 3 tables.