Densities and viscosities of aqueous solutions of ethanolamine (MEA), 1-butyl-3-methylimidazolium tetrafluoroborate (BmimBF(4)), and their mixtures were measured. The experiments covered the following mole fraction ranges: 1.00-9.38 mol % of BmimBF(4) in its aqueous solutions, 3.94-21.97 mol % of MEA in its aqueous solutions, and 1.32-17.01 mol % of MEA + 5.10-34.02 mol % of BmimBF(4) in the mixtures. The results are compared with available data in the literature and correlated using a semiempirical formula as functions of the temperature and concentration of MEA and BmimBF(4). At the concentration suitable for the CO2 capture, the viscosity of the hybrid absorbent did not significantly increase by the addition of BmimBF(4). Moreover, the addition of BmimBF(4) reduced the Henry's law constant, which is significant for reaction kinetics.