The viscosity of aqueous NaCl solutions with dissolved CO2 was measured at conditions representing an underground aquifer at a depth of (1000 to 2000) in for the geological storage of CO2 (i.e., (30 to 60) degreesC and (10 to 20) MPa at a mass fraction of NaCl between 0 and 0.03 by using a sedimenting solid particle type viscometer with an estimated uncertainty of +/-2%). On the basis of this experimental data, an empirical equation for predicting this viscosity as a function of the temperature and mole fraction of CO2 for these conditions was derived.