The flash photolysis of o-naphthil was studied in the temperature range 77-320 K. Flash excited a-naphthil produces two triplet conformers and two types of hydrogen-adduct radical in the presence of a hydrogen donor. In isopropanol, one triplet conformer absorbs at 525 nm (triplet I) and the other at 430 nm (triplet II). Triplet I present in the rigid glassy medium, remains in equilibrium with triplet II at the melting point of the soft glass. In fluid medium, only triplet II is present. The enthalpy change for the triplet I to triplet II equilibrium process is 12.4 W mol-'. In fluid medium, an equilibrium of the precursors of the two types of radicals can be determined from the relative absorption ratios of radical I at 550 nm and radical II at 460 nm immediately after the laser flash at various temperatures. The equilibrium of the radical precursors requires an enthalpy change of 43.2 kJ mol-'. The equilibria between the two triplet conformers and the two types of radical precursor are not observed in hydrocarbon medium; however, the triplet (530 nm) present in the solid glassy hydrocarbon at low temperature is different from that (440 nm) present in the liquid hydrocarbon. The hydrogen-adduct radicals are generated directly from the excited singlet state when hydrogen donor triethylamine is present in excess. The decay kinetics, stability and reactivity of the different conformers are discussed in the light of the puckered and planar conformations of the o-naphthil molecule.