The reaction enthalpy for O atom transfer from Cp*ReO3 to PPh(3) in toluene solution is measured to be -28.4 +/- 0.3 kcal/mol and -28.0 +/- 0.6 kcal/mol in THF solution. This allows the strength of the Re=O bond in Cp*ReO3 (in solution) to be estimated at 116.8 +/- 1.2 kcal/mol.