The thermal decomposition of di-tert-butyl peroxide has been studied in the presence of high pressures of cyclopropane; the rate constant for the decomposition was determined from the sum of the rates of formation of tert-butanol and acetone. In the temperature range 125-160 OC, i.e. above the critical point of cyclopropane, concentrations of cyclopropane of between 2.6 and 3.6 mol/l (60-80 amagats) were used; in the temperature range 100-124 "C, i.e. below the critical point of cyclopropane, an equivalent mean (liquid + gas) cyclopropane concentration was used. An Arrhenius plot of the rate constants for the peroxide decomposition is continuous through the critical point of the cyclopropane, and is indistinguishable from the best Arrhenius line representing all previous work on the reaction.However, an Arrhenius plot of the ratio of the rates of formation of tert-butanol and acetone shows a marked discontinuity at the critical point, but further investigation shows that this results from the unequal distribution of the reactant peroxide between the two phases which are present below the critical point of the cyclopropane.Less extensive series of experiments using propane, isobutane, cyanogen, and carbon dioxide as pressurizing gases confirm the pressure-independence of the rate of decomposition of di-tert-butyl peroxide: the experiments with propane and isobutane confirm the phase-independence of the rate; the propane experiments also exhibit the same apparent kinetic discontinuity at the critical point. Finally, rate-constant data are obtained for the reactions of the tert-butoxy radical with cyclopropane and isobutane, and for the addition of methyl radicals to cyanogen.La dCcomposition thermique du pCroxyde di-tert-butylique a Ct C CtudiCe en prCsence de fortes pressions de cyclopropane; la constante de vitesse de dCcomposition a Ct C dCterminCe a partir de la somme des vitesses de formation du tert-butanol et de I'acetone. Dans un intervalle de ternpkrature de 125-160 "C, i.e. au-dessus du point critique du cyclopropane, des concentrations de cyclopropane entre 2.6 et 3.6 mol/l (60-80 amagate) ont Ctt utilisCes; dans I'intervalle de temperature 100-124 "C, i.e. en-dessous du point critique du cyclopropane, une moyenne Cquivalente (liquide + gaz) de concentration de cyclopropane a Ct C utilisCe. Un graphe d'ArrhCnius des constantes de vitesse pour la dicomposition du pCroxyde est continu en passant par le point critique du cyclopropane, et est indiscernable de la meilleure ligne dYArrhCnius reprksentant tous les travaux antickdents sur la riaction.Cependant, un graphe d'ArrhCnius du rapport des vitesses deformation du tert-butanol et de l'acCtone montre une discontinuitt marquee au point critique, mais des recherches ultkrieures montrent que ceci rCsulte d'une distribution inkgale du rCactif pCroxyde entre les deux phases qui sont prisentes au-dessous du point critique du cyclopropane.Des sCries d'experiences moins Ctendues utilisant le propane, I'isobutane, le cyanogene et le dioxyde de carbone comme gas po...