The thermal reaction of triethyl phosphite with carbon tetrachloride in the dark, in vacuo, has been shown to be heterolytic. In the presence of added azobisisobutyronitrile (AIBN) at least part of the product [(EtO)₂P(O)CCl₃] is formed homolytically. The reaction of the phosphite, carbon tetrachloride, and butane-1-thiol is (a) ionic in the presence of t-butylcatechol, (b) homolytic in the presence of AlBN, and (c) has both ionic and homolytic components in the absence of additives. The corresponding reactions of bromotrichloromethane are less clearly defined and both radical and ionic pathways may be followed. In hexane, the thermal reaction of bromotrichloromethane with the phosphite is ionic, whereas in the presence of AlBN it is radical, at least in part.