Bereczky et al. have measured the particulate matter (PM) emissions from a diesel engine when oxygenates are added to the fuel . The mass concentration of PM in the exhaust ΓPM was reduced by 23% when 16 mol% of a methyl ester biodiesel (BD) was added to a conventional diesel fuel (D), and by 36% when 14 mol% of the BD and 15 mol% of 4-5-L were added to the D. We estimate YSIs for each of these fuel mixtures in SI I. Figure that there is a reasonably linear relationship between the measured ΓPM and the estimated YSIs. This observation indicates that the differences in YSI observed in this study between the lactones and conventional fuels are likely to translate into reductions in PM emissions from engines. The YSIs of the lactones depend strongly on the molecular structure of the molecule. As an example, Fig. shows how the YSI of 4-5-L changes when its structure is altered in various ways. Removal of the methyl side chain produces 4-4-L and reduces the sooting tendency by 17.7 YSI units (left side of the figure). Extension of the side chain to an ethyl group produces 4-6-L and increases the YSI, but only by 6.1 (right). The comparisons to the molecules in the top row are particularly interesting since they have the same number of carbon atoms as 4-5-L. Ring-opening isomerization produces 4PA and only changes the YSI slightly, by +2.4 (upper left). However, rearrangement of the side-chain carbon into the ring produces 5-5-L and reduces the YSI by 13.8 (upper middle). Most strikingly, addition of a double carbon-carbon bond to the ring produces α-A-L and reduces the YSI by 16.7 (upper right)-this is one of very few cases where a greater degree of unsaturation corresponds to a lower sooting tendency . Overall, these results show that while all lactones have lower sooting tendencies than conventional fuels, the specific choice of lactone has a large impact on the soot reduction benefit. Of particular significance, 4-5-L is the lactone that has been discussed most as a fuel, but its isomer 5-5-L is a better choice from the viewpoint of reducing soot emissions.