The lowest-lying triplet and singlet potential energy surfaces for the O(P-3) + CH2=C=CH2 reaction were theoretically characterized using the complete basis set model chemistry, CBS-QB3. The primary product distributions for the multistate multiwell reactions on the individual surfaces were then determined by RRKM statistical rate theory and weak-collision master equation analysis using the exact stochastic simulation method. The results predict that the electrophilic O-addition pathways on the central and terminal carbon atom are dominant up to combustion temperatures. Major predicted end-products for the addition routes include CO + C2H4, (CH2)-C-3 + H2CCO, and CH2=C-center dot-CHO + H center dot, in agreement with experimental evidence. CO + C2H4 are mainly generated from the lowest-lying singlet surface after an intersystem crossing process from the initial triplet surface. Efficient H-abstraction pathways are newly identified and occur on two different electronic state surfaces, (3)A" and 3A', resulting in OH + propargyl radicals; they are predicted to play an important role at higher temperatures in hydrocarbon combustion chemistry and flames, with estimated contributions of ca. 35% at 2000 K. The overall thermal rate coefficient k(O + C3H4) at 200-1000 K was computed using multistate transition state theory: k(T) = 1.60 x 10(-17) x T-2.05 x exp(-90 K/ T) cm(3) molecule(-1) s(-1), in good agreement with experimental data available for the 300-600 K range.