The reactions between cis-[RuH2(DPPE)(2)] and a number of acids in THF solution (DPPE = Ph2PCH2CH2PPh2) show biphasic kinetics, with initial formation of trans-[RuH(H-2)(DPPE)(2)](+) followed by slower substitution of coordinated dihydrogen by the anion of the acid. The formation of the dihydrogen complex is a second-order process that occurs with an inverse kinetic isotope effect and rate constants k(HX) strongly dependent on the nature of the acid. There is a linear correlation between the values of log k(HX) for cis-[RuH2(DPPE)(2)] and the related cis-[FeH2(PP3)] [PP3 = P(CH2CH2PPh2)(3)] that leads to two parameters, S and R, that can be used as a measure of the selectivity and intrinsic reactivity of the dihydride toward acids. The possible contributions to the values of these parameters are discussed, especially the role of the isomerization of the starting complex and the basicity of the reacting species. The substitution of coordinated dihydrogen in trans-[RuH(H-2)(DPPE)(2)](+) occurs through a simple dissociative mechanism instead of the more complicated one previously proposed far substitutions in the analogous Fe complex; the mechanistic change is associated with the relative strength of the M-H-2 and M-P(chelate) bonds.