Vapor-liquid equilibrium data were obtained for the 1,4-dimethylbenzene + 1,3-dimethylbenzene binary system and the 1,4-dimethylbenzene + 1,3-dimethylbenzene + 1,2-dimethylbenzene ternary system at the isobaric conditions of 6.66 and 26.66 kPa. The activity coefficients in the binary system were found to be thermodynamically consistent, and the results were equally well correlated with the Margules, Van Laar, Wilson, NRTL, and UNIQUAC equations. The binary interaction parameters obtained from this and our previous studies are used to predict the vapor-liquid equilibrium for the ternary mixture using the above models.