Homogeneous WGS catalysis employing Rh4(CO)12 as precursor in aqueous pyridine (py) was investigated in a batch reactor at 80-degrees-C, P(CO) = 1 atm, and [Rh] = 0.02 M. The WGSR has the highest activity in py containing 3% H2O, and can be protracted to the complete consumption of water. [L(x)H][Rh5(CO)13(py)2](L = H2O, py) was found to accumulate during, the catalysis, while protracted WGSR leaves [Rh(CO)n(py)4-n][Rh5(CO)13(py)2] (n = 1, 2) in dehydrated py. Separate experiments confirmed that in anhydrous py at 80-degrees-C [(py)2H][Rh5(CO)13(py) 2] gives H-2 and [Rh(CO)n(py)4-n][Rh5(CO)13(py)2], while in wet py at 30-degrees-C the latter rhodium species is reduced by CO back to [L(x)H][Rh5(CO)13(py)2]. With CO2 formation. Hence, a catalytic cycle is proposed where cis-[Rh(CO)2(py)2]+ and [Rh5(CO)13(py)2]- are the active species, and where H-2 and CO2 production accompanies their mutual interconversion. Direct investigation of the catalytic reaction showed an inhibiting effect by chloride ions at <5% H2O. The reaction resulted to be acid cocatalyzed up to a limiting behavior. Both these aspects are explained on the basis of the chemical properties of the two proposed active species, which were elucidated by separate experiments. While a systematic study was not attempted, a TOF(H2) = 172 (T = 80-degrees-C, P(CO) = 1 atm, [Rh] = 0.02 M) was achieved in py containing 3% H2O when a chloride-free precursor is employed in the presence of added acid (CF3SO3H/Rh = 10).