The rate constant (k(1)) for the reaction of Cl atoms with C2H4 has been measured as a function of pressure (0.2-100 Torr) at 297 K using the relative rate technique in two reactors with either FTIR or GC analysis. The results of these and previous experiments (100-3000 Torr) can be described to within +/-10% by a Tree expression with the limiting rate constants k(1)(infinity) = (3.2 +/- 0.15) x 10(-10) cm(3) molecule(-1) s(-1); and k(1)(0) = (1.42 +/- 0.05) x 10(-29) cm(6) molecule(-2) s(-1) using F-cent = 0.6. The stated uncertainties are statistical only, and the true uncertainties in the limiting rate constants must include uncertainty in F-cent. Temperature-dependent (297-383 K) measurements of k(1) were carried out in the low-pressure regime (0.2-2 Torr) yielding the rate constant expression k(1)(0) = (1.7 +/- 0.3) x 10(-29)(T/298)(-3.28) cm(3) molecule(-1) s(-1). The rate constant, k(1b), for the abstraction channel of reaction 1 to form the vinyl radical was determined at 300, 343, and 383 K. This measurement of k(1b) in combination with literature values of k(-1b) allows a determination of the heat of formation of the vinyl radical by the third law method [Delta H(f,298)degrees(C2H3) = 70.6 +/- 0.4 kcal/mol]. Using the same data with the second law yields Delta H(f,298)degrees(C2H3) = 69.6 +/- 1.6 kcal/mol. These measurements agree satisfactorily with a recent negative-ion photoelectron spectroscopy determination of this quantity (71.6 +/- 0.8 kcal mol(-1)). The pressure dependence of the rate constant (k(2)) for the reaction of Cl with C2H2 was determined over the range 0.3-700 Torr. The results of these and previous experiments (100-6000 Torr) can be described to within +/-10% by a Tree expression with the limiting rate constants k(2)(infinity) = (2.0 +/- 0.1) x 10(-10) cm(3) molecule(-1) s(-1) and k(1)(0) = (6.1 +/- 0.2) x 10(-30) cm(6) molecule(-2) s(-1) using F-cent = 0.6.