Two general mechanisms of sensitized delayed fluorescence in solution are proposed. Both mechanisms require, as an initial stage, the transfer of triplet energy from the sensitizer to the acceptor. In one mechanism the excited singlet of the acceptor is then formed by self-quenching of acceptor triplets; in the other it is formed by mixed triplet quenching between donor and acceptor triplets. Evidence is adduced for the occurrence of the mixed triplet mechanism in the delayed fluorescence of naphthacene sensitized by anthracene. The naphthacene is found to undergo photosensitized decomposition. The possibility of stable mixed dimers formed by mixed triplet quenching is discussed.