The reaction of [Cp*RuCl](4) (1, Cp* = eta(5)-C5Me5) with the carbene ligand 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene (IMes) affords a coordinatively unsaturated Cp*Ru(IMes)Cl (3) complex in 86% isolated yield. Solution calorimetric results in this system provide information concerning the electron donor properties of the carbene ligand, which are very similar to those of PCy3. Structural information from single-crystal X-ray studies for complex 3 allows the determination of steric parameters associated with this ligand. The thermochemical information is used to predict the magnitude of the enthalpic driving force behind substitution reactions involving RuCl2(=C(H)Ph)(PCy3)(2) (1) and the carbene ligand, IMes, affording the RuCl2(=C(H)Ph)(PCy3)(IMes) (6) complex in high yield. A similar mixed carbene/phosphine ruthenium complex, RuCl2(=C(H)Ph)(PPh3)(IMes), can also be isolated from RuCl2(=C(H)Ph)(PPh3)(2) and the IMes ligand. A single-crystal X-ray diffraction study has been performed on 6. The thermal stability of these mixed phosphine/carbene ruthenium carbene complexes has been studied at 60 degrees C in toluene. Their catalytic activity has been evaluated for the ring closing metathesis of diethyldiallylmalonate.