The effect of the addition of a third ion to the ionic liquid 1-butyl-3-methylimidazolium acetate [C(4)C(1)Im]-[OAc] was studied through the measurement of the enthalpy of mixing and of the excess molar volume of its mixtures with 1-butyl-3-methylimidazolium trifluoroacetate [C(4)C(1)Im]-[CF3CO2], 1-butyl-3-methylimidazolium bis-(trifluoromethylsulfonyl)imide [C(4)C(1)Im][NTf2], and tetrabutylphosphonium acetate [P-4444][OAc]. Negative enthalpies of mixing (Delta H-mix < 0) and positive excess molar volumes (V-E > 0) were observed in all cases. The infrared and NMR studies of the pure ionic liquids and their mixtures show that the presence of a third ion with a weaker affinity with the common counterion contributes to prevailing the more favorable hydrogen-bond, herein always between the imidazolium cation and the acetate anion. Both radial and spatial distribution functions calculated by molecular simulation confirm this behavior. The remarkable enhancement of the viscosities of the [C(4)C(1)Im][OAc] + [P-4444][OAc] mixtures could be discussed in light of the calculated friction coefficients.