Water-soluble ruthenium complexes [(eta(6)-arene)Ru(kappa(2)-L)](n+) (n = 0,1) ([Ru]-1- [Ru]-9) ligated with pyridine-based ligands are synthesized, and the molecular structure of the representative complex [Ru]-2 is confirmed by X-ray crystallography. The studied complexes are employed for the catalytic dehydrogenation of formic acid in water. Screening of these complexes inferred that [Ru]-1 [(eta(6)-C10H14)RU(kappa(2)-NpyOH-L1)Cl](+) (L1 = pyridine-2-ylmethanol) outperformed others with an initial turnover frequency of 1548 h(-1). Complex [Ru]-1 also exhibited high stability in water and can be recycled up to seven times with a total turnover number of 6050. In addition to formic acid dehydrogenation, [Ru]-1 also catalyzed the conversion of formaldehyde to hydrogen gas in water under base-free conditions. The effects of temperature, pH, formic acid, and catalyst concentration on the reaction kinetics are investigated in detail. Mass and NMR based mechanistic investigations inferred the presence of several important intermediate species, such as ruthenium-formate species [Ru]-1B and ruthenium-hydride species [Ru]-1C, involved in the catalytic dehydrogenation reaction. Moreover, the molecular structure of a diruthenium species [Ru]-1A' is also authenticated by single-crystal X-ray crystallography.