The selective dehydrogenation of aqueous solutions of HCOOH/HCOONa to H2 and CO2 gas mixtures has been investigated using RuCl3·3H2O as a homogeneous catalyst precursor in the presence of di␣erent monoaryl-biaryl or alkyl-biaryl phosphines and aryl diphosphines bearing sulfonated groups. All catalytic systems were used in water without any additives and proved to be active at 90 °C, giving high conversions and good TOF values. As an alternative Ru(II) metal precursor, the known dimer [Ru( 6-C6H6)Cl2]2 was also tested as in situ catalyst with selected phosphines as well as an isolated Ru(II)-catalyst with one of them. By using high-pressure NMR (HPNMR) techniques, indications on the nature of the active species involved in the catalytic cycles were obtained.