The ultimate objective of the present research program is to develop a predictive reaction model for soot formation in hydrocarbon flames. The principal accomplishments in the reported period are 1 A computer algorithm was developed that calculates optical properties of an ensemble of particles whose size distribution is given in terms of moments of the size distribution function. 2 A computational study of sooting limits in laminar premixed flames was initiated and completed. It was found that the critical equivalence ratios for soot appearance, both the absolute values and temperature dependencies, can be predicted fairly close to the experimental observations. 3 New estimation techniques were developed and applied for calculations of standard-state enthalpies of formation and binary gaseous diffusion coefficients of polycyclic aromatic hydrocarbons PAHs and their radicals, thus providing critical information for accurate modeling of soot formation in flames. 4 Theoretical studies of a bench-mark ion-molecule reaction were initiated and completed. The results obtained further support the neutral-species reaction pathway as the predominant route for the formation and growth of PAHs, the precursors to soot in hydrocarbon flames. 5 A computational study of pressure effect on soot formation was performed. 6 A reduced model for PAH and soot formation in turbulent reactive flows was developed.