The kinetics of the reaction of phenyl radical with ethylene has been investigated with the cavity-ring-down method at six temperatures between 297 and 523 K under a constant pressure of 20 torr Ar. A test performed at 60-tort pressure revealed no noticeable change in the measured rate constant value. The second-order rate constant determined by directly monitoring the decay of the phenyl radical under excess ethylene concentration conditions could be effectively represented by the Arrhenius equation k~2 H ~ = 10 11.925-11.3Sexp( _ 2,250 _+ 630/T) cm3/s, where the errors represent one-standard deviation evaluated with the weighting factor w i = (ki/oi)2. This low-temperature and comparatively high-pressure result can be satisfactorily correlated by means of the RRKM theory with the high-temperature (1000-1300 K) and low-pressure (1-10 mtorr) styrene formation data reported by Fahr and Stein (Ref. 15), k~6HsC2H3 = 4.2 × 10 12exp(-3120/T) cm3/s. The result of our multichannel RRKM calculation based on the mechanism C6H5 + C2H4 a C6H5CH2CH2 t b C6H5C2H3 + H --a c C6 H5C2 H4 ( + M) suggests that the rate constant for the production of styrene under the conditions employed by Fahr and Stein (k b) is essentially the same as the total rate constant, k~2n4 = k b + k c, because k b ~ k c at high temperatures (T > 1000 K) and low pressures (P < 20 torr). Under atmospheric combustion conditions, however, both k b and kc are comparable and strongly dependent on T and P. The total rate constant for the C6H 5 + C2H 4 reaction can be given by the following expression: k~.2H 4 = 1.2 × 10-lVTl62exp(-1490/T)cm3/s for the temperature range 300-2000 K, effectively encompassing both sets of kinetic data.