The catalytic activity of the zwitterionic complex [(sulphos)Rh(cod)] for the hydrogenation and hydrogenolysis reactions of benzo[b]thiophene (BT) has been studied in either methanol or liquid-biphase systems comprising MeOH or MeOH-H2O as the polar phase and n-heptane as the organic phase [sulphos = -O3S(C6H4)CH2C(CH2PPh2)(3)]. The catalyst activity is independent of the phase variation. Under neutral conditions, the slow but selective hydrogenation of BT to 2,3-dihydrobenzo[b]thiophene is observed. Conversely, in the presence of NaOH or other strong bases, the fast and selective hydrogenolysis of BT to 2-ethylthiophenol sodium salt occurs. In a typical liquid-biphase hydrogenolysis reaction [35 mg (0.035 mmol) of catalyst, 470 mg (3.5 mmol) of BT, 180 mg (4.5 mmol) of NaOH, 5 mt of MeOH, 5 mL of H2O, 10 mL of n-heptane, 30 bar of H-2, 160 degrees C], all the substrate is practically consumed in ca. 5 h to give the 2-ethylthiophenolate product. The strong base plays a dual role in the hydrogenolysis reaction : it promotes the formation of the Rh-H species (which is necessary for the C-S insertion step) by heterolytic splitting of H-2 and accelerates the conversion of BT by aiding the reductive elimination of the hydrogenolysis product. The effect of the H-2 pressure and of the substrate, catalyst, and base concentrations on the conversion rate of BT has been studied under liquid-biphase conditions. High-pressure NMR experiments in sapphire tubes have provided mechanistic information on the catalysis cycle for the hydrogenolysis of BT in MeOH. Under catalytic conditions, the phosphorus-containing rhodium compounds (visible on the NMR time scale) are the trihydride [(sulphos)RhH3](-) and the dihydride 2-ethylthiophenolate complex [(sulphos)Rh(H)(2)(o-S(C6H4)C2H5)](-). Consistent with previous studies, the reductive elimination of the thiol from the metal center is suggested to be the rate-determining step of the hydrogenolysis reaction of BT catalyzed by the 16e(-) fragment [(sulphos)RhH](-).