Aqueous solutions of cesium formate and bicarbonate repre- sent an effective hydrogen storage–delivery couple that under- goes either release or take up of hydrogen in the presence of {RuCl2(mTPPTS)2}2 (TPPTS = triphenylphosphine trisulfonate) and excess mTPPTS ligand, with no other additives required. Cesium salt solutions offer the advantage of improved volu- metric and gravimetric H2 density compared to their sodium and potassium analogs, owing to their high water solubility. Details of the equilibrium between formate and bicarbonate, which constitutes an important parameter for the applicability of this H storage/release cycle, were determined. H production is readily tunable by controlling the operating pressure. This behavior was also rationalized through the identification of catalytic intermediates under various conditions. High con- centration formate and bicarbonate solutions were used during the tests and the bidirectional catalytic system could be recycled without loss of activity or replacement of solvent. A tentative mechanism is proposed for the formate dehydrogen- ation step. Among the identified hydride species, the penta- coordinated [RuH(H2O)(TPPTS)3] complex was indispensable for promoting the formate dehydrogenation reaction.