Relative permittivity and density on mixing from T = 288.15 K to 328.15 K and atmospheric pressure have been measured over the whole composition range for CH3O(CH2CH2O)mCH(3) polyoxyethyleneglycol dimethyl ether with m = 4 (also called 2,5,8,11,14-pentaoxapentadecane or tetraglyme) + (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 of volume fractions. Arguments are put forward that support the choice of the definition in terms of mole fraction. The Redlich-kister equation has been used to estimate the binary fitting parameters and standard deviations from the regression lines were calculated The density and excess molar volumes were fitted as a function of temperature and mole fraction to a polynomial equation. The temperature dependence of derived magnitudes, such as the isobaric thermal expansion coefficient, alpha, (partial derivative V-m(E)/partial derivative T)(p,x), and (partial derivative H-m(E)/partial derivative P)(T,x) were computed, due to its importance in the study of specific molecular interactions. Different traditional mixing rules have been applied to predict the permittivity of these mixtures. (C) 2007 Elsevier Ltd. All rights reserved.