The photochemical decomposition of t-butyl hydroperoxide by light of wave-length 3130 angstrom has been investigated in three solvents. Reaction mechanisms are elucidated by consideration of the products and the quantum yields of decomposition. In carbon tetrachloride a chain reaction occurs in which the quantum yield of 3$\cdot $2 at 20 degrees C increases to 5$\cdot $3 at 50 degrees C. The main products are t-butyl alcohol and oxygen with smaller amounts of acetone, water and compounds arising from the oxidation of methyl radicals. The same series of reactions takes place in n-hexane, but superimposed are oxidation reactions involving solvent molecules which ultimately lead to the formation of alcohols. The quantum yield in this solvent is 3$\cdot $9 and independent of temperature. When the peroxide is irradiated in dioxan solution immediate hydrogenation of the radicals produced in the primary photo-chemical act prevents the formation of reaction chains and the quantum yield is unity. The interaction of the radicals with solvent molecules is such that some of the etheric oxygen of the dioxan is transformed into alcoholic hydroxyl during the course of the reaction, and the fragmentation of dioxan gives formaldehyde. Experiments with a dioxan solution using light of wave-length 2450 to 2800 angstrom show no fundamental change in the mode of decomposition of the peroxide, but an increase in concentration of the products of dioxan decomposition indicates a more vigorous attack by the radicals on the solvent.