The rates of reactions of ClzP atoms with methane, and with a series of substituted methanes, have been measured using mass spectrometric analysis of molecular reactant consumption in the presence of excess Cl atoms, with pseudo first order kinetic analysis ; this approach minimizes secondary reactions and the problem of reaction stoichionietries. The following rate constants were obtained for CI + RH reactions : l~g~~( k c~~/ c m~ molecule-' s-I) = (-10.294+0.043)-(778k 16) KIT, 300-686 K ; l o g l o ( k~~s~i / c m 3 molecule-' s-l) = (-9.673 k 0.087) -(776 & 30) K / T , 300-604 K ; l~g~~( k c~~~i~/ c m~ molecule-' s-I) = (-10.078 k0.074)-(629& 26) K / T , 298-621 K ; l~g~~( k c~~~~/ c m~ molecule-' s-I) = (-10.480+0.051)-(599k 19) KIT, 297-652 K ; l~g~~( k c~~c i~/ c m~ moIecuIe-' s-I) = -12.64+ 0.08, 652 K. The correlations of these results with data from competitive chlorination studies are discussed in relation to the uncertainty surrounding the Arrhenius parameters for the primary reference reaction, Values for the kinetic isotope (H/D) rate constant ratios, (~C H J~C D~) and ( k c~c i~ /~C D C I~) , were C1+ HZ+HCl+ H. measured over wide ranges of temperature : loglo(kCHs/kCD4) = (-0.155f0.024)+(384+ 12) K / T , 298-1018 K ; 298-1006 K. lOglo(kCHCl~/kCDC13) = (-0.014+0.020)+(167If:9) K / T , These results are in agreement with the predictions of the semi-empirical BEBO (bond-energy-bond order) method.Chlorination reactions with molecules RH, involving abstraction of an H atom, and leading to formation of HCl and a radical R, constitute one of the largest series of elementary reactions for which relative (although not absolute) rate constants have been measured. These kinetic data have provided some of the most extensive results for the testing of theories of absolute reaction rates, including kinetic isotope effects. Examples include the reactions : Cl+H,, D2, HT l * ; Cl+ CH4, CD4, CzH6, CzD6.2 However, surprisingly few direct measurements of absolute values for rate constants are a~ailable.~ All the reactions in the series C1+CH4, CH,Cl, CH2C12, CHC13, have activation energies less than 20 kJ f110l-l,~ whilst the values for -AU,",, in these exothermic reactions gradually increase in this series from about 4 kJ mol-l for Cl+ CH4, to about 42 kJ mol-1 for C1+ CHC13.4 Measurements of the ratios of rate constants (ICR*H/ICR"H) for reactions of this type have been carried out using competitive methods, with C1 atoms generated either thermally or photolytically from C12 : kWH Cl + R'H+R' + HCI, C1+ R"H+R" + HCl. 1547 7kRttn9