Kinematic viscosities of binary and multicomponent mixtures containing poly(ethylene glycols) were measured as a function of temperature. Viscosities of the binary mixtures were used to calculate the excess molar Gibbs energy of activation for viscous flow G(*E). The G(*E) values are positive over the entire composition range and they increase as the difference in the molecular masses of the two polymers increases. The group-contribution viscosity model GC-UNIMOD was employed to correlate the viscosity of the binary systems and then to predict the viscosities of the multicomponent mixtures. Average absolute deviation around 3.5% was obtained for the viscosity calculations using the GC-UNIMOD model.