The ruthenium(II) complex fac-[Ru(CO)2(H2O)3(C(O)C2H5)][CF3SO3] dissolved in aqueous tetrabutylammonium hydrogensulfate ([(CH3(CH2)3)4N][HSO4]) or sodium hydrogensulfate (NaHSO4) catalyzes the hydrocarboxylation of ethylene to propionic acid and additionally produces minor amounts of hydrocarbonylation products (diethyl ketone and propanal), under water-gas shift reaction conditions. This system is stable with a selectivity of 90% to propionic acid for high ethylene conversion. A turnover frequency of propionic acid, TOF(C2H5CO2H)/24 h=5x103 (TOF(C2H5CO2H)=([(moles of C2H5CO2H)/(moles of Ru)xrt)]x24 h) was achieved for Ru=7.45x10-4 mol, [(CH3(CH2)3)4N][HSO4]=80 g (2.36x10-2 mol); H2O=40 g (2.22 mol); CO=C2H4=20 g (total pressure=88 atm); T=150°C by a reaction time (rt) of 2.87 h. The countercation (sodium or tetrabutylammonium), the ruthenium concentration and the hydrogensulfate/ H2O ratio of the medium affect the catalytic reaction. A nonlinear dependence on total ruthenium concentration was shown. The data are discussed in terms of a potential catalytic cycle. Formation of propionic acid comes from hydrolysis, and formation of diethyl ketone and propanal comes from hydrogenolysis of the Ru-ketyl and Ru-acyl complexes, respectively.