The elucidation of the WGSR promoted by ruthenium carbonyls in acidic media started with the detection of the Ru(0), Ru(I), and Ru(II) intermediate complexes, namely Ru(3)(CO)(12), Ru(2)[&mgr;-eta(2)-OC(CF(3))O](2)(CO)(6), and fac-[Ru(CF(3)COO)(3)(CO)(3)](-), which accumulate when CF(3)COOH is employed as an acid cocatalyst. Under catalytic conditions, the three were found to interconvert through elementary steps which produce CO(2) and H(2). In fact, Ru(0) is oxidized by H(+) to Ru(I) and half the hydrogen of the catalytic cycle is supplied by this reaction. On the other hand, Ru(I) disproportionates to Ru(0) and Ru(II), and this latter species undergoes nucleophilic attack by H(2)O. The decomposition of the metallacarboxylic acid intermediate gives back Ru(I), while H(2) and CO(2) are produced in a 1/2 molar ratio. The two alternating pathways for dihydrogen formation, namely Ru(0) oxidation by H(+) and the decomposition of a metallacarboxylic acid intermediate, involve H(2) reductive elimination from the same RuHCF(3)COO(CO)(2)L(2) intermediate (L = H(2)O, ethers). These findings define an acid-cocatalyzed WGSR whose distinctive features are (i) the intervention of a disproportionation reaction to generate a Ru(II) electron poor complex, whose CO ligands can undergo nucleophilic attack by water, (ii) the generation of the hydrido intermediate for dihydrogen production through two distinct reaction patways, and (iii) the reductive elimination of H(2) from the hydrido intermediate without involving H(+) from the medium.