The density and viscosity of multicomponent solutions play an important role in industrial areas concerning mass and heat transfer, as well as fluid flow. To expand the potential application of ILs, it is important to know the physicochemical properties of ILs with solvents for ILs' widely used. The density and viscosity for binary mixtures of ionic liquid (IL) [C(2)py][DCA] with ethanol, 1-propanol and 2-propanol were measured across the entire range of mole fraction (x(1) = 0.0000-1.0000) at T = (288.15-318.15) K. The excess molar volumes, V-E, were calculated and well fitted by Redlich-Kister equation. The obtained VE are negative values over the whole composition range and it means that there is greater interaction between the ions and the alcohol molecules. The values of Delta eta have been applied to description of the viscosity deviation of the mixtures from the ideal one, but the equation of excess Gibbs energy of activation for viscous flow, Delta G(not equal E), for mixture cannot be derived from Delta eta. And more serious is that an incorrect equation of Delta G(not equal E) is widely used in the literature. In view of this situation, we put forward a new concept - the relative viscosity of mixture, eta(r). On a basis of the new concept, a correct equation of Delta G(not equal E) is obtained. The values of eta(r) and Delta G(not equal E) of mixtures [C(2)py][DCA]+alcohol over the whole composition range were calculated. The values of eta(r) are less than one and Delta G(not equal E) are negative so that this result means that the interaction among ions becomes weaker due to ion solvation. The Delta G(not equal E) values of the mixtures [C(2)py] [DCA]+alcohol were well fitted by the Redlich-Kister equation. Using the Redlich-Kister's parameters, the values of viscosity for mixtures [C(2)py][DCA]+alcohols were estimated and the estimated eta(est.) is in good agreement with the experimental g. In order to test the reliability of the method proposed in this work, we also deal with the data of mixtures for [HDBU]IM + H2O, [BDBU]IM + H2O and [Mim]Ac + Alcohols in the literatures. The results also prove that this method is reasonable. (C) 2017 Published by Elsevier Ltd.