Abstract X-ray diffraction analyses of a mixture of dipalmitoylphosphatidylcholine (DPPC) and 5α-pregnanedione (9:1 molar ratio) in water indicated the presence of two lamellar phases. High resolution calorimetry of dilute solutions of the same mixture in greater than 99.5 wt% water indicate only a slight change in the thermogram when compared to pure DPPC bilayers. Increasing the amount of sterol in the mixture did not alter the thermogram significantly. These results indicate that 5α-pregnanedione acts in a similar manner to cholesterol when present in low molar percent mixtures with DPPC and in less than 70 wt% water. As the water content is significantly increased, the sterol is no longer associated with the bilayer in contrast to similar studies done with cholesterol in DPPC bilayers.