The question has been studied whether a metastable triplet state of acetylene is formed during its mercury photosensitized reaction, using biacetyl as a probe and employing a modulation-phase shift technique. While no emission is observable in the mercury photosensitized reactions of acetylene or biacetyl alone, in the presence of both compounds biacetyl phosphorescence is generated whose intensity, under suitable conditions of pressure and flow, approaches closely the one of benzene photosensitized biacetyl emission. The phenomenon is not caused by products such as benzene and can be quenched by increasing the acetylene pressure. A mechanism explaining the emission as energy transfer from a metastable acetylene triplet state to biacetyl is proposed, and rate parameters are evaluated by comparing this model with the experimental results. Self-quenching of triplet acetylene occurs at 0.083 times the rate of energy transfer to biacetyl. The excitation energy of triplet acetylene, E, was estimated to be 2.6<E<4.7 eV on the basis of the biacetyl phosphorescence yield and quenching experiments with other triplet states. In addition, the results provide an estimate, viz., 2×1011 M-1- sec-1, for the biacetyl triplet-triplet annihilation rate constant.