Di-tert-butyl peroxide has been photolyzed at 2537 Å in the gaseous phase in the presence of up to 47 amagats (2.10 mol/l) of propane and of cyclopropane. It was confirmed that no acetone is formed in the limit of infinite hydrocarbon concentration and therefore that the primary chemical act leading to the eventual formation of acetone is the formation of two tert-butoxy radicals from the excited peroxide molecule; in addition, some crude information was obtained concerning relative rates of photochemical vs. deactivation processes. It was also found that at these densities the tert-butoxy radical formed in the photolysis of di-tert-butyl peroxide did not appear to differ in chemical reactivity from the tert-butoxy radical formed in the thermal decomposition of di-tert-butyl peroxide.