This paper presents a DFT study of hydrogen exchange in the binuclear polyhydride complex CpRu-(μ-H)4RuCp (1) with molecular hydrogen. Both dissociative (through the CpRu(μ-H)2RuCp intermediate) and associative mechanisms (through hydrogen coordination to yield intermediates Cp2Ru2H6 and subsequent hydrogen dissociation) are considered. The calculations show that the dissociative pathway has a prohibitively high barrier for hydrogen dissociation, with ΔG‡ 298 about 34 kcal·mol-1. The associative mechanism is much more favorable. There are, in turn, two possible pathways, going through either cis-Cp(H)2Ru(μ-H)2Ru(H)2Cp (2a) or trans-Cp(H)2Ru(μ-H)2Ru(H)2Cp (2b). The cis pathway is more favorable, with ΔG‡298 barriers for the rate-determining step of 24 kcal·mol-1. The intermediate 2a can either dissociate directly or undergo isomerization processes involving interchanging of bridging and terminal hydrides. Overall, the calculations support the associative pathway for the hydrogen exchange, but show that the details of the mechanism are rather complicated.