The viscosities (eta) and refractive indices (n(D)) were measured for the binary mixtures of o-cresol + PEG 200/ PEG 300/PEG 400 over the whole composition range at five different temperatures (288.15, 293.15, 298.15, 303.15, and 308.15 K). From the experimental results, the viscosity deviation (Delta(eta)), the excess Gibbs free energy of activation of viscous flow (Delta G*(E)), the deviation in refractive index (Delta(phi)n(D)), and the molar refraction (R-m) have been calculated. The excess or deviation properties were fitted with Redlich-Kister polynomial equation and their coefficients and standard deviations were calculated. The positive and negative excess or deviation properties were discussed in terms of specific molecular interactions between mixture components. The excess or deviation property values (Delta(eta), Delta G*(E), and Delta(phi)n(D)) follow the order PEG400 > PEG300 > PEG200. On the basis of the viscosity data, Gibbs energies of viscous flow (Delta G*), enthalpy of activation of viscous flow (Delta H*), and entropy of activation of viscous flow (Delta S*) were also calculated.