The crystal structures of n-nonylammonium dihydrogen phosphate and n-octylammonium dihydrogen phosphate were determined by X-ray crystallography. Lattice potential energies of the compounds were obtained from the crystallographic data. The molar enthalpies of dissolution of the two compounds at various molalities in the double-distilled water were measured at T = 298.15 K by means of an isoperibol solution-reaction calorimeter, respectively. According to the Pitzer's electrolyte solution theory, the standard molar enthalpies of dissolution of the two compounds at infinite dilution were determined to be 34.50 kJ.mol(-1) and 22.32 kJ.mol(-1), and the Pitzer's parameters beta((0)L)(ML) and beta((1)L)(MX) were obtained. The values of apparent relative molar enthalpies (L-Phi) of the title compounds and relative partial molar enthalpies ((L) over bar (2) and (L) over bar (1) of the solute and the solvent at different concentrations were derived from the experimental values of the enthalpies of dissolution of the two compounds.