The multiplicities of the excited states resulting from the thermal 1 and photochemical 2 decomposition of tetramethyl-1,2-dioxetane have recently been shown to result in the selective and high yield production of the (n,v*) triplet state of acetone. We report in this note both chemical and spectroscopic evidence for the formation of triplet acetone from the pyrene singlet sensitized decomposition of tetra3 methyl-1,2-dioxetane (TMD) . The observation that both 9,10-diphenylanthracene and 9,10-dibromoanthracene act as efficient singlet sensitizers of the photochemical decomposition Of cis-diethoxy-1,2-dioxetane has already been made. 4 Product studies for such a process, however, have not as yet been reported. Addition of TMD to acetonitrile solutions of pyrene resulted in the quenching of the pyrene fluorescence intensity. Stern-Volmer treatment of the data led to straight line plots of IO/I vs [TMDI with kqr = 335 i: 29 (least squares analysis). Using the value of 352 + 15 nsec for the pyrene lifetime under our conditions (measured via single photon counting), resulted in a value of kq = 9.52 f 1.44 x lo8 M-lsec-‘. A similar value of k = 9.11 f 0.51 x lo* was obtained via Stern-Volmer q analysis of fluorescence lifetime quenching as measured by single photon counting. In order to establish whether excited acetone molecules are produced in a pyrene sensitized TMD decomposition, a chemical titration was designed. The substrate for the chemical titration must meet two standards. It should not quench the excited