A bimolecular rate constant (k(q)) and an efficiency (OCTET) of an endoergonic triplet energy transfer from triplet alpha-naphthyl phenyl ketone (NPK) to alpha-naphthylmethyl phenyl sulfide (NMPS) in solution, where the energy difference (DeltaE(T)) is 1.7 kcal mol(-1), have been determined by means of nanosecond laser flash photolysis. Base on the Arrhenius plots of the k(q) value for the NPK-NMPS system, an activation energy (DeltaE(a)) for the endoergonic triplet energy transfer is found to be 2.6 kcal mol(-1), which is larger than the endothermic energy gap (DeltaE(T) = 1.7 kcal mol(-1)). The endoergonic triplet energy transfer in solution is demonstrated to proceed with the aid of thermal activation for the first time. (C) 2002 Elsevier Science B.V. All rights reserved.