A mechanistic investigation has been of the thermolysis of several endoperoxides of anthracenes and naphthalenes which produce molecular oxygen and the parent aromatic species quantitatively. Qualitative thermochemical measurements in the solid state indicate that in all the cases studied, the reactions were endothermic. This situation appears to be valid in solution also. Clean first-order kinetics were observed for these thermolyses. Activation parameters were derived from the temperature dependence of the first-order rate constants. The primary yields of singlet molecular oxygen (/sup 1/O/sub 2/) from the several endoperoxides were determined, and a correlation was discovered between the A factors (..delta..S/sup + +/ values) for thermolysis and the yield of /sup 1/O/sub 2/. It was found that high A factors (positive ..delta..S/sup + +/ values) correlated with relatively low yields of /sup 1/O/sub 2/, and that low A factors (slightly negative or near zero ..delta..S/sup + +/ values) correlated with nearly quantitative yields of /sup 1/O/sub 2/. These two results are interpreted in terms of a diradical mechanism which leads to low yield of /sup 1/O/sub 2/ and a concerted mechanism which leads to quantitative yields of /sup 1/O/sub 2/, respectively. This interpretation is consistent with the observation of a magnetic fieldmore » effect on the yield of /sup 1/O/sub 2/ from endoperoxides whose thermolyses proceed with positive ..delta..S/sup + +/ values and the absence of a magnetic field effect on the yield of /sup 1/O/sub i/ endoperoxides whose thermolyses proceed with near zero ..delta..S/sup + +/ values. Further support for the occurrence of a diradical mechanism is available from the demonstration of a special /sup 17/O isotope effect on the thermolysis of an endoperoxide which is postulated to undergo thermolysis principally via a diradical intermediate. The thermolysis of endoperoxides which decompose mainly by a diradical mechanism yields triplet molecular oxygen that is selectively enriched in /sup 17/O.« less