Complex formation reaction between benzophenone ketyl (BPK) radical and triethylarnine was investigated by means of transient absorption and time-resolved ESR methods. The formation constants (K-f's) in benzene, toluene, o-xylene, and anisole were measured by a transient absorption method. The enthalpy and entropy changes upon the complex formation were determined by van't Hoff plots of K-f values around 300 K. The DeltaG values in the complex formation reaction at 298 K were calculated from these thermodynamic parameters, which show notable difference from those in aliphatic solvents and chlorotoluenes previously reported. The BPK conformation in these solvents was investigated by analyzing the time-resolved ESR spectra of BPK and by the DFT calculation. The hyperfine coupling constant of hydroxy-proton (a(OH)) was found to depend on solvent as well and there exists a clear correlation between the a(OH) and K-f (or DeltaG) values. It was concluded that the conformation of BPK, especially the conformation between OH and two phenyl groups, depends on the solvation and is an important factor to control the complex formation reaction.