Rate constants for the reactions of chlorine atoms with n-butane and simple alkenes, as well as most of their deuterated analogs, were studied at room temperature using two independent techniques. Using a fast flow discharge system (FFDS), the decay of chlorine atoms in 1 Ton He was followed using resonance fluorescence at 135 nm. In relative rate (RR) studies the decay of the organic was followed, relative to a reference compound, using GC-FID when they both reacted with chlorine atoms. These RR measurements were performed at 1 Ton in N-2 and at I atm in both N-2 and air. The results of the FFDS and RR studies at I Ton were generally in excellent agreement with each other and, where available, with literature data. Discrepancies appear to exist at 1 atm for isoprene, where the addition portion of the rate constant measured in this laboratory is 40% higher than a value for k(infinity) reported recently by Bedjanian et al.(40) Our rate constant for 1,3-butadiene is 25% smaller than that of Bierbach et al.(39) Rate constants measured in these studies which have not been previously reported in the literature are as follows (in units of cm(3) molecule(-1) s(-1), with the errors being the statistical 2 sigma errors) : C3D6 (4.1 +/- 0.8) X 10(-11) in I Torr He, (4.3 +/- 1.0) x 10(-11) in 1 Ton Nz, and (2.3 +/- 0.3) x 10(-10) in 1 atm N-2 or air; 1-C4H8 (1.0 rt 0.1) x 10(-10) in 1 Ton : He, (1.2 +/- 0.2) x 10(-10) in 1 Ton N-2, and (2.2 +/-0.3) x 10(-10) at I arm Nz or air; 1-C4D8 (1.0 +/- 0.2) x 10(-10) in I Ton He, (1.2 +/- 0.2) x 10(-10) in I Ton Nz, and (2.0 +/- 0.4) x 10(-10) in 1 atn N-2 or air; n-C4D10 (1.6 +/- 0.1) x 10(-10) averaged over all pressures and carrier gases. Deuteration results in a normal kinetic isotope effect (KIE) for direct hydrogen abstraction, but an inverse kinetic isotope effect for addition to the double bond. The KTE (k(H)/k(D)) for n-butane was measured to be 1.4 +/- 0.2. For ethene, an inverse KIE was measured, 0.74 +/- 0.06 at 1 atm in Nz or air compared to an average of 0.35 at 1 Ton in NZ or He reported in earlier studies.(29) The KIEs for the larger alkenes were unity within experimental error. For propene at 1 Ton, the inverse KIE for addition is largely counterbalanced by a normal KIE for abstraction of an allylic hydrogen. For the larger alkenes, this result is consistent with expectations because addition is close to the high-pressure limit even at 1 Ton and abstraction is expected to play a minor role in the overall reaction. The atmospheric implications of these measurements are discussed.