Cavity ring-down (CRD) spectroscopy and ab initio calculations have determined the reaction rate coefficients, mechanism, and thermochemistry relevant to the addition of a chlorine atom to allene. Chlorine atoms were produced by laser photolysis at 351 nm and the addition reaction products were probed at a variable delay by CRD spectroscopy using a second laser pulse. Ab initio results indicate that the only persistent addition product is the 2-chloroallyl (C3H4Cl) radical. We measured the continuum spectrum of the 2-chloroallyl radical between 238 and 252 nm and determined the absorption cross section, sigma(240)(C3H4Cl) = (2.5 +/- 0.5) x 10(-17) cm(2). By fitting the C3H4Cl absorption data to complex kinetic mechanisms, rate coefficients at 298 K were found to be k(Cl+C3H4; 656 Pa, N-2) = (1.61 +/- 0.27) x 10(-10) cm(3) molecule(-1) s(-1), k(Cl+C3H4; 670 Pa, He) = (1.34 +/- 0.24) x 10(-10) cm(3) molecule(-1) s(-1), and k(Cl+C3H4; 1330 Pa, He) = (1.75 +/-0.25) x 10(-10) cm(3) molecule(-1) s(-1). The rate coefficient of the self-reaction displayed no pressure dependence between 434 and 1347 Pa in N-2 buffer giving k(C3H4Cl+C3H4Cl) = (3.7 +/- 1.0) x 10(-11) cm(3) molecule(-1) s(-1). A study of the addition reaction of 2-chloroallyl radical and oxygen molecule determined sigma(240) (C3H4ClO2) = (3.6 +/- 0.7) x 10(-18) cm(2) and k(O-2+C3H4Cl, 705 Pa N-2) = (3.6 +/- 0.4) x 10(-13) cm(3) molecule(-1) s(-1). The listed uncertainties denote two standard deviations, and those for rate coefficients include the uncertainty of the appropriate absorption cross section.