Analysis of previously reported experimental results has shown that the formation of diheptylperoxide from n-heptane in a motored C.F.R. engine, although it involved free radicals, was not a chain reaction. It was possible to calculate an activation energy which was really the sum of the activation energy of the two steps in the formation of diheptylperoxide. The formation must have been strongly exothermic on thermodynamic grounds and this fitted the proposed reaction mechanism involving free radicals. Only a small proportion of the oxygen in the charge may have been available to form peroxides. A reaction scheme was proposed for the surface initiation, this involved active sites which were alternately reduced and reoxidised.