As described in Section 3.3 we modeled the kinetics of ISC in these systems by applying NA-TST in its microcanonical form. By using MESMER software, we calculated high-pressure limiting ISC rate coefficients for each triplet species; these are given in Table . These rate coefficients agree qualitatively with the experimental branching ratios shown in Figure and illustrate the relative ISC rates between the different systems. For example, the computed rates are very close for the ehtene and propene-derived species, whereas in the triple-bond adducts the methyl group leads to a 5-10-fold rate increase, consistently with a significant increase in the yield of singlet products in propyne relative to acenylene. The modelled highpressure conditions correspond to ultrafast vibrational relaxation, often observed in condensed phase. Interestingly, in the experiments studying oxygen addition to double-bond species 2 in liquid nitrogen or in solid films the fragmentation products have not been observed. This is consistent with our mechanism: once a triplet intermediate is formed, it is cooled down, making dissociative pathways energetically inaccessible while activating ISC. We note that under combustion conditions this thermal, high-pressure-limit picture can be misleading. In reality, the Cvetanović diradicals are formed with excess internal energy and the ISC (and other reactive channels on the triple surface) compete with collisional energy transfer with the bath.