Cationic organic dyes (phenosafranine, safranine T (ST+), thiopyronine, and methylene blue) are efficient quenchers of the triplet states of aromatic ketones. The triplets of benzophenone (BP), xanthone, thioxanthone, benzil, and N-methylacridone are quenched by the dyes via energy transfer, generating triplet excited dyes (typically k(q) = 5.5 x 10(9) M(-1) s(-1) for (BP)-B-3* + ST+ in acetonitrile solution). Regarding Michler’s ketone, p-(dimethylamino)benzophenone, and 3,6-bis(dimethylamino)thioxanthone, the mechanism of triplet quenching is solvent-dependent. Electron transfer occurs in polar solvents like acetonitrile, and energy transfer is the dominant reaction pathway in less polar solvents, such as dichloromethane. Delayed fluorescence of the dye caused by heterotriplet-triplet annihilation involving ketone and dye was detected upon studying the reaction of the dyes with triplet excited ketones (BP, xanthone, thioxanthone, and benzil). By applying two successive laser pulses (lambda(ex) = 532 and 308 nm, respectively), the rate constant of the reaction (BP)-B-3* + (3)(ST+)* was determined : k(ht) = 1.1 x 10(10) M(-1) s(-1).