We investigated the influence of some estradiol derivatives on the thermal and structural properties of binary mixtures of cholesteryl enantate and cholesteryl myristate. The characterization of the mesophases was made by polarizing optical microscopy, differential scanning calorimetry and X-ray diffraction. A considerable shift of the phase transition temperature ICh, as well as the temperature of formation of the SmA* phase was observed. Besides phase sequences and phase transition temperatures the measurements enabled the determination of some molecular parameters of the mixtures: the thickness of smectic layers and the longitudinal spacing distance in the cholesteric phase (d) and the average distance between the long axes of neighboring molecules (D).