AbstractDirect determinations have been made of the heat of substitutive chlorination (R.H + Cl2–→R.Cl + HCl) for benzene, chlorobenzene, toluene,cyclohexane, n‐hexane, ethylene dichloride, and chloroform. Heats of formation are derived for 1 : 1 : 2‐trichloroethane, chlorocyclohexane, and benzyl chloride.Similarly the heat of additive chlorination of three chloroethylenes has been determined, and an approximation to the heat of additive chlorination of benzene has been made.The results are shown to agree closely with the values which can be calculated from recently published heats of combustion of the relevant compounds. Both for substitutive and for additive chlorination, the heat evolution tends to diminish markedly as the number of chlorine atoms already in the molecule is in