Relative permittivities of the binary liquid mixtures {(CH3O)(2)CO or C5H10O3} + CH3 (CH2)(7)CH3 at temperatures from 288.15 K to 308.15 K and atmospheric pressure, have been determined over the whole composition range. Values of relative permittivity were measured using a HP4284A precision LCR meter together with the measuring cell HP16452A at frequency 1 MHz. Relative permittivity increments were determined from experimental data and fitted to a variable-degree polynomial function. Their parameters and root mean square deviations were calculated. Different mixing rules were used for predicting the permittivity of these mixtures. The predictions are better when the volume change on mixing is considered.