IN a recent communication on the recombination of chlorine atoms Linnett and Booth1 reported a value of 3 × 1014 cm6 moles-2 sec-1 for the third-order rate constant defined by: The method used by these workers determines only the relative atom concentration along a tube. It is hence suitable for rate constant determinations when the process is first order in atom concentration. In this case, however, since the process is second order in Cl-atoms the absolute concentration must be known. Linnett and Booth could only `assume' arbitrary values for the absolute concentrations. It is our belief that these assumed values were wrong by an order of magnitude.