Excitation of15NO molecules into the B2π state by collisional intramolecular energy transfer from the metastable a4π state was studied under single-collision conditions. Passing a beam of15NO (a4π) through a cell filled with argon at a few mTorr, B2π → ×2π emission was observed. It originated from only three specific B state levels, namely2π1/2,J= 21·5 and2π3/2,J= 21·5 inv= 2 and2π1/2,J= 10·5 inv= 3. This selectivity is characteristic of a gateway-type process mediated by spectroscopic perturbations. The analysis showed that in the last case there is a direct spin-orbit interaction of the emitting B state level with a well-specified level of NO(a). The other two perturbations are caused by interactions of the respective NO(B) levels with uniquely identifiable levels of the short-lived NO(b4Σ−) state, which in turn are collisionally populated from a broad range of NO(a4π) levels. This latter mechanism is thus a novel type of gateway process. The results explain the anomalous spectral distribution observed in an earlier isotopic study of the15N16O afterglow, which showed an enhanced population of the vibrational levels B,v= 2 and 3 [14].