The heats of formation of radicals in solution were measured by using transient absorption and time-resolved thermal lensing methods. The heat of formation of benzophenone ketyl radical (BPK) in benzene solution was determined to be 21 +/- 2 kcal/mol. Using this value, the heats of formation of several radicals, which were generated as the counter radicals of BPK in the hydrogen abstraction reaction of triplet benzophenone, were also determined. The solvation energies of radicals are compared with those of their parent molecules and discussed in light of solvation mechanism. (C) 1998 Elsevier Science S.A. All rights reserved.