AbstractThe use of iodine monochloride (ICl) as a thermal source of chlorine atoms in known concentration is discussed with particular reference to the suppression, by large excesses of iodine, of the chain processes normally associated with chlorine atom reactions. The kinetics and mechanism of the reaction of ICl with hydrogen are presented in a study covering the temperature range 205–337°C, and the pressure ranges: ICl, 6–20 torr; I2, 3–13 torr; and H2, 9–520 torr. The reaction, followed spectrophotometrically in a static system, is shown to be homogeneous, first order in ICl and in H2, and inverse half‐order in I2, over several half‐lifetimes of the ICl, yielding HCl as the sole product. The rate data obtained in this work for the reactionare combined with the critically evaluated results of other workers in an Arrhenius plot covering the temperature range 286–730°C, and three orders‐of‐magnitude in the rate constant, yielding the results, logk1/(1/mole sec) = 10.68–5.26/θ, where θ = 2.303RTin kcal/mole. This value ofk1is lower by a factor of about two than that proposed in a recent review by Fettis and Knox, and is clearly at variance by a factor of two or more with the most recent da