The excited states of UV-B absorber (4-methylbenzylidene)camphor (MBC) have been studied through measurements of UV absorption, phosphorescence, triplet triplet (T T) absorption, and steady-state and time-resolved electron paramagnetic resonance spectra in ethanol. The energy level and lifetime of the lowest excited triplet (T-1) state of MBC were determined. The energy level of the T-1 state of MBC is much lower than that of photolabile 4-tert-butyl-4'-methoxydibenzoylmethane. The weak phosphorescence and strong time-resolved EPR signals, and T T absorption band of MBC were observed. These facts suggest that the significant proportion of the lowest excited singlet (S-1) molecules undergoes intersystem crossing to the T-1 state and the deactivation process from the T-1 state is predominantly radiationless. The quantum yields of singlet oxygen production by MBC determined by time-resolved near-IR luminescence measurements are 0.05 +/- 0.01 and 0.06 +/- 0.01 in ethanol and in acetonitrile, respectively. The photostability of MBC arises from the (3)pi pi r* character in the T-1 state. The zero-field splitting parameters in the T-1 state are D = 0.0901 cm(-1) and E = 0.0498 cm(-1) . The sublevel preferentially populated by intersystem crossing is T-y (y close to in-plane short axis and to the C=O direction).