The kinetics of the reaction (mu-H)(2)Ru-3(CO)(8)(mu-P(t-Bu)(2))(2) + H-2 reversible arrow (mu-H)(2)Ru-3(CO)(8)(H)(2)(mu-P(t-Bu)(2))(2) have been studied. The reaction of (mu-H)(2)Ru-3(CO)(8)(mu-P(t-Bu)(2))(2) With H-2 has a rate law which is first-order in cluster concentration and in hydrogen pressure and inverse order in CO pressure; on the basis of the rate law, activation parameters, and deuterium kinetic isotope effect, hydrogen addition is proposed to involve rapid, reversible dissociation of a carbonyl ligand, followed by rate-determining oxidative addition of hydrogen through a three-center transition state at a single metal atom. Loss of hydrogen from (mu-H)(2)Ru-3(H)(2)(CO)(8)(mu-P(t-Bu)(2))(2) also involves reversible loss of a carbonyl, followed by rate-determining reductive elimination of molecular hydrogen. The reaction is highly sensitive to the steric bulk of the phosphido substituents, as (mu-H)(2)Ru-3(CO)(8)(mu-PR(2))(2), R = cyclohexyl and phenyl, do not react with hydrogen. In addition, the rate of exchange with (CO)-C-13 is much faster for R = t-Bu than for R = cyclohexyl. Based upon the temperature dependence of the equilibrium constant for hydrogenation, the energy for the unbridged Ru-Ru bond of (mu-H)(2)Ru-3(CO)(8)(mu-P(t-Bu)(2))(2) is estimated to be 47-59 kJ/mol, the low value being attributed to steric strain.