Reaction enthalpies of (COD)Ru(allyl)2 (COD = η4-1,5-cyclooctadiene; allyl = 2-methylpropenyl) with a series of bidentate phosphines (dppm, dppf, dppe, dppb, dppp, depe, dmpe) have been measured by anaerobic solution calorimetry. The relative stability of the resulting complexes is strongly influenced by the electronic donor properties of the bidentate phosphine ligand. Reactions involving ligands that are better σP) donors result in higher enthalpy values and, therefore, more thermodynamically stable complexes. Additionally, the synthesis and characterization of two new ruthenium allyl complexes Ru(allyl)2(dppf) (3) and Ru(allyl)2(depe) (8) and the X-ray crystal structure of 3 are reported.Key words: ruthenium, allyl, solution calorimetry, thermodynamics, X-ray structure.