The structure of two N-alkylribonamides both in the anhydrous state and in concentrated water solution has been studied by X-ray diffraction. A Lamellar Crystalline, a Lamellar Liquid-Crystalline and a Cylindrical Hexagonal Liquid-Crystalline Structures have been found. The influence of temperature and of water concentration on the geometrical parameters of the mesophases has been established. The organization of the molecules in the three structures is described and a step by step melting behavior is proposed for the N-alkylaldonamides.