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-torr 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"C2H4 = 10- 11.92± 0.35 exp (-2,250 ± 630/T) cm3/s where the errors represent one-standard deviation evaluated with the weighting factor wi = ( ki σi)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″C6H5C2H3 = 4.2 × 10-12exp(-3120/T)cm3/s. The result of our multichannel RRKM calculation based on the mechanism C6H5 + C2H4 a ⇌ C6H5CH2CH2 b → C6H5C2H3 + H c → C6H5C2H4 +(M) suggests that the rate constant for the production of styrene under the conditions employed by Fahr and Stein (kb) is essentially the same as the total rate constant, k″C2H4 = kb + kc, because kb ≫ kc at high temperatures (T > 1000 K) and low pressures (P < 20 torr). Under atmospheric combustion conditions, however, both kb and kc are comparable and strongly dependent on T and P. The total rate constant for the C6H5 + C2H4 reaction can be given by the following expression: k"C2H4 = 1.2×10- 17 T 1.62 exp (-1490/T) cm3/s for the temperature range 300-2000 K, effectively encompassing both sets of kinetic data.