Fig. shows the measured YSIs of several terpenes and the corresponding hydrogenated terpenes with two hydrogen atoms added (dihydroterpenes) and four hydrogen atoms added (tetrahydroterpenes). In each case the YSI decreases from the original terpenes to the dihydroterpenes, and then some more to the tetrahydroterpenes. Therefore, hydrogenation of the C=C bonds (terpene → dihydroterpene) both mitigates soot and improves oxidative stability . Hydrogenation of rings (dihydroterpene → tetrahydroterpene) also mitigates soot but potentially lowers the gravimetric energy density associated with the ring strain. a The uncertainty is ± 6%; b The YSIs of these compounds were corrected for impurities based on their measured composition in SI A and the assumption that the YSI of a mixture is the mole-fraction-weighted average of the YSIs of each component.