The laser photolysis/infrared long path absorption technique has been used to study the reaction of Cl atoms with C2H4. The time-resolved population of HCl produced in the reaction is monitored by continuous infrared absorption after pulsed photolytic generation of Cl atoms. At room temperature, the reaction proceeds principally through addition to form a chloroethyl radical, and the HCl production at 10 Torr is almost exclusively via secondary reactions. Above 500 K the abstraction channel dominates the HCl production, allowing direct extraction of Arrhenius parameters for the hydrogen abstraction, The abstraction rates are extracted by correcting the total rate by the HCl yield, which is determined by comparison with the Cl + C3H8 reaction. The abstraction reaction is well-described between 500 and 800 K by the simple Arrhenius expression k(T) (6.2 +/- 1.4) x 10(-11)e(-(3400+/-450/T) cm(3) molecule(-1) s(-1) (2 sigma). The activation energy determined in the present work would yield a heat of formation of the vinyl radical of Delta H degrees(f,298)(C2H3) = 71.3 +/- 1.3 kcal mol(-1) when combined with an extrapolation of published 298-495 K measurements of the HCl + C2H3 activation energy. A third-law analysis al 500 K yields Delta H degrees(f,298)(C2H3) = 71.6 +/- 0.3 kcal mol(-1).