The reactions shown in Figure do not explicitly include the non-reaction path, in which excited βRO 2 reforms βR and O 2 (though this is taken into account implicitly by an overall decrease in reaction rate). This non-reaction can be important when determining how much of βR will β-scission to form isobutene instead of adding O 2 , which the model by Merchant et al. had trouble accurately predicting. Figure in the ESI † shows the fraction of excited βRO 2 which decomposes back to the βR. For the β-network at high temperatures, over 95% of excited βRO 2 re-forms βR + O 2 , in agreement with the results in Sun et al. Given the small region showing the formation of acetone, CH 2 O, and OH in Figure , the Waddington pathway appears less important than has been suggested previously. This is due to sensitivity in the kinetic rates and the masking of secondary products in Figure . The fraction going to this path is highly dependent on the barrier heights. A sensitivity study in which the barrier height of β-γHO2FromRO2 was raised by 10 kJ/mol, to correspond to that of Sun et al., increased the range of conditions where the Waddington pathway was most dominant (Figure of the ESI † ). In addition, in the region around 800 K and 50 bar, where βR + O and stabilized βRO 2 are essentially reversible, the major secondary pathway is the Waddington path.