The crystal structures of ephedrine hydrochloride and N-methylephedrine hydrochloride have been determined by X-ray crystallography. The molar enthalpies of dissolution of the two compounds with various molalities were measured at T = 298.15 K in distilled water by means of isoperibol solution-reaction calorimeter, respectively. According to the theory of Pitzer, the molar enthalpies of dissolution of the two compounds at infinite dilution, Delta H-s(m)infinity, and Pitzer's parameters beta((0)L)(MX), beta((1)L)(MX), and C-MX(phi L), were obtained. Then the values of apparent relative molar enthalpies (L-phi), hydration heat, and relative partial molar enthalpies of the two compounds ((K) over bar (2)) were derived from the experimental results of the enthalpies of dissolution of the two compounds. Finally, the ionic radius of the cations can be calculated from the data of the corresponding effective volume of the cations. (C) 2009 Elsevier Ltd. All rights reserved.