ReaxFF MD simulations were performed to understand the decomposition of the lactones in pure pyrolysis at high temperatures. These conditions are a good approximation to the soot formation region in the nonpremixed YSI flames . A key benefit of MD simulations is that they do not require a pre-existing chemical kinetic mechanism. This capability is critical since mechanisms are currently available for only one lactone, 4-5-L . In this study we simulated three lactones to examine the effects of ring size (4-4-L vs. 5-5-L) and the consequence of a side chain (4-5-L vs. 4-4-L). Figure presents the results for the three lactones: 4-4-L (top panel), 5-5-L (middle), and 4-5-L (bottom). Each panel shows the number of molecules of the fuel and of the six main products as a function of time during the simulation. In all three cases, the reactant lactone (grey lines) is consumed over a timescale of 0.5 to 3 ns. The major products are alkenes (butenes, C4H8; propene, C3H6; and ethylene, C2H4;) and oxygen-containing species (CO2; CO; and formaldehyde, CH2O). Significantly, CO2 (green lines) is the main oxygen-containing product for all three lactones, and its level is close to the maximum possible (40 molecules). This result explains the observation in Fig. that lactones have large sooting tendencies relative to other dioxygenated hydrocarbons. In general, the decomposition reactions of lactones can either produce CO2 and a regular hydrocarbon (R1), or two oxygenated hydrocarbons (R2):