The reactions of chlorine with hydrogen and hydrocarbons have been widely studied and it is well established that they are chain re¬ factions involving very long chains. There is general agreement as to the chain initiating and propagating steps but still considerable di¬ sagreement as to the chain terminating steps. The modern trend in kinetics has been away from the establishment of mechanism in detail and towards the determination of Arrhenius parameters for elementary reactions such as those involved in chain propagation. Hie present work was aimed at determining accurately the rate constants for the attack of chlorine atoms on different hydrocarbons (reaction(1)) Cl + R-H -> R + H-Cl (1) and on different positions in the same hydrocarbon. This has been accomplished by employing a competitive technique in which reaction products are analysed for products characteristic of the initial chlorine at era attack. Absolute rate constants for each hydrocarbon are then found, if the absolute value for at least one of the substances studied is known. In this work the rate constant for the chlorine atom attack on hydrogen, which is reasonably well established, has been used as the standard value and all other absolute rate constants are obtained from this. The activation energies obtained from the rate constants have been explained in a qualitative manner, while the A factors have been used to test transition state theory.