The Viscosities of TiO2 -water nanofluids are obtained experimently, which are much greater than the calculated values of the existing viscosity formulations of suspensions. By analyzing the solvation effect of the solid/liquid interfacial layers, a concept of effective volume fraction is introduced into the usual viscosity formulations, the correction equations of which can correctly predict the magnitude of the viscosities of TiO2-in-water nanofluids.