A novel compound n-octanammonium oxalate (C8H17NH3)(2)C2O4(s) was synthesized by the method of liquid phase reaction. Crystal structure of n-octanammonium oxalate was determined by X-ray crystallography. The lattice potential energy of the compound was calculated to be U-POT = 1050.90 kJ . mol (1). Molar enthalpies of dissolution of the compound at different concentrations m/(mol . kg (1)) were measured by an isoperibol solution-reaction calorimeter at T = 298.15 K. According to the Pitzer's electrolyte solution theory, the molar enthalpy of dissolution of the compound at infinite dilution (Delta H-sol(m)infinity) and Pitzer parameters (beta((0)L)(MX) and beta((1)L)(MX)) were obtained. The values of apparent relative molar enthalpies (Phi(L)) of the title compound 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 experimental values of enthalpies of dissolution. The enthalpy change of the synthesis reaction of n-octanammonium oxalate was determined to be Delta H-r(m)Theta = -(153.68 +/- 0.50) kJ . mol(-1) by an isoperibol solution-reaction calorimeter, and standard molar enthalpy of formation of (C8H17NH3)(2)C2O4(s) was calculated to be Delta H-f(m)Theta = -(1463.6 +/- 2.4) kJ . mol(-1) by using the enthalpies of dissolution and other auxiliary thermodynamic quantities. (c) 2012 Elsevier Ltd. All rights reserved.