The potential energy surface for the O(P-3) + C2H4 reaction, which plays an important role in C2H4/O-2 flames and in hydrocarbon combustion in general, was theoretically reinvestigated using various quantum chemical methods, including G3, CBS-QB3, G2M(CC,MP2), and MRCI. The energy surfaces of both the lowest-lying triplet and singlet electronic states were constructed. The primary product distribution for the multiwell multichannel reaction was then determined by RRKM statistical rate theory and weak-collision master equation analysis using the exact stochastic simulation method. Intersystem crossing of the "hot" CH2CH2O triplet adduct to the singlet surface, shown to account for about half of the products, was estimated to proceed at a rate of approximate to 1.5 x 10(11) s(-1). In addition, the thermal rate coefficients k(O + C2H4) in the T = 200-2000 K range were computed using multistate transition state theory and fitted by a modified Arrhenius expression as k(T) = 1.69 x 10(-16) x T-1.66 x exp(-331 K/T). Our computed rates and product distributions agree well with the available experimental results. Product yields are found to show a monotonic dependence on temperature. The major products (with predicted yields at T = 300 K/2000 K) are: CH3 + CHO (48/37%), H + CH2CHO (40/19%), and CH2((XBj)-B-3) + H2CO (5/29%), whereas H + CH3CO, H-2 + H2CCO, and CH4 + CO are all minor (<= 5%).