Relative permittivity and density on 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 = 4 (also called 2,5,8,11,14-pentaoxapentadecane or tetraglyme) + (n-nonane or n-dodecane)}. Relative permittivity increments and excess molar volumes on mixing for the above systems have been calculated. 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 the mote fraction and temperature to a polynomial equation. The temperature dependence of derived magnitudes, as isobaric thermal expansibility, alpha, (partial derivative V-m(E)/partial derivative T)(P,x) were computed, due to its importance in the study of specific molecular interactions. Different mixing rules have been applied to predict the permittivity of these mixtures and the results indicate that the predictions are better when the volume change on mixing is incorporated in calculations. The work concludes with an interpretation of the sign of the permittivity increment and of the behaviour with temperature of that permittivity increment and of excess molar volume. (C) 2005 Elsevier Ltd. All rights reserved.