The gel-liquid crystal phase transitions of the pseudo-binary systems of cephalins DMPE and DHPE in excess water were studied by differential scanning calorimetry. The phase diagram of the pseudo-binary systems has been given. The experiments showed that the partial phase separation in gel phase might occur at least at the mole fractions of DHPE below 0.1. The analysis by the model of ideal solution showed that both the cephalins were non-ideally miscible both in the gel phases and in the liquid crystal phases. The analysis by the model of regular solution showed that all the non-ideality parameters in the gel phases were larger than those in the liquid crystal phases at the same temperature. All the non-ideality parameters were not constant, but rather dependent on temperature. The gel-liquid crystal phase transitions of the pseudo-binary systems of cephalins DMPE and DHPE in excess water were studied by differential scanning calorimetry. The phase diagram of the pseudo-binary systems has been given. The experiments showed that the partial phase separation in The analysis by the model of ideal solution showed that both both the cephalins were non-ideally miscible both in the gel phases and in the liquid crystal phases. The analysis by the model of regular solution showed that all the non-ideality parameters in the gel phases were larger than those in the liquid crystal phases at the same temperature. All the non-ideality parameters were not constant, but rather dependent on temperature.