The 1:1 complex formation between benzophenone ketyl radical (BPK) and triethylamine in various solvents was studied by means of nanosecond laser flash photolysis. The complex shows an absorption band slightly red-shifted from that of uncoupled BPK. The formation constants (K-f＇s) were determined from the absorbance change of the complex against amine concentration. The K-f values are in the range 60-103 M-1 depending on the dielectric constant of solvent (epsilon(r) = 10.7-1.89). Enthalpy and entropy changes in the complex formation reactions were determined from van＇t Hoff plots around room temperature. The measured enthalpy change of -2.0 to -7.5 kcal mol(-1) depends on epsilon(r), which is explained with an Onsager＇s reaction field theory. The measured entropy change varies from -16.5 cal K mol(-1) to nearly zero, and the dependence on the solvent is discussed in terms of the solvation effect.