Fluorescence lifetime and triplet-state spectral, kinetic, and energetic properties are reported for the first time for a perylenebis(dicarboximide) dye, N,N'-bis(2,5-di-tert-butylphenyl)-3,4,9,10-perylenebis(dicarboximide) (DBPI). The fluorescence lifetime (3.8 +/- 0.2 ns), quantum yield (greater than or equal to 0.95), and singlet-state energy (54 +/- 1 kcal mol/sup -1/) were determined for DBPI in a variety of organic solvents (24/sup 0/C). Triplet sensitization was used to characterize the triplet state of DBPI by laser flash photolysis and pulse radiolysis. The triplet-triplet absorption spectrum of DBPI had a maximum in the range 495-505 nm with an extinction coefficient of (6.3 +/- 0.7) x 10/sup 4/ M/sup -1/ cm/sup -1/, and the triplet-state energy and lifetime were 27.5 +/- 2 kcal mol /sup -1/ and approx. = 100 ..mu..s, respectively. Direct photoexcitation produced the DBPA triplet with very low quantum yields (<0.001). Excimerlike fluorescence spectral and lifetime data for DBPI in the solid state are also presented.