We confirm a report that 3,3-dimethyldioxetane (1a) produces 2-methyl-1,2-propanediol (5a) when heated in 1,4-cyclohexadiene as solvent. previously, the resulting diol was taken as evidence for a stepwise, biradical mechanism in which a 1,4-dioxy biradical is trapped by the H-atom donor solvent. To investigate this possibility in more detail, the decomposition rates and product yields were measured under various conditions, We have found that the diol is not formed by trapping of the 1,4-dioxy biradical, but rather from an initial attack by the double bond of 1,4-cyclohexadiene on the strained O-O bond of the dioxetane. Furthermore, we report that tetramethyldioxetane (1b) also produces 1.6 +/- 1.0% of the corresponding diol, pinacol (5b), when heated in 1,4-cyclohexadiene. As the 1,4-dioxy biradical is not a likely intermediate in the thermal decomposition of dioxetanes, we support the previously proposed asynchronous, concerted mechanism for thermal decomposition of dioxetanes; the definitive evidence is formation of n-pi* versus pi-pi* triplets.