Relative permittivity and density oil mixing from T = (288.15 to 328.15) K and atmospheric pressure have been measured over the whole composition range for {CH3O(CH2CH2O)(m)CH3 polyoxyethyleneglycol dimethyl ether with m = 3 (also called 2,5,8,11, tetraoxadodecane or triglyme) + (dimethyl or diethyl carbonate). For these systems, the deviation of permittivity, Delta epsilon, changes sign depending on whether it is defined on the basis of mole fractions or volume fractions. Arguments are put forward that support the choice of the definition in terms of mole fraction. The density and excess molar volumes as a function of temperature and mole fraction were fitted to a polynomial equation. The temperature dependence of derived quantities, such as the isobaric thermal expansion coefficient, x, (partial derivative V-m(E)/partial derivative T)(Px) and (partial derivative H-m(E)/partial derivative P)(Tx) were computed, due to their importance in the study of specific molecular interactions. The excess molar volumes were analyzed using the Prigoginwe-Flory-Patterson (PFP) theory of solution thermodynamics. Different traditional mixing rules were applied to predict the permittivity of the mixtures.