Potassium hydrogen phthalate (KHP) was synthesized by the method of liquid-phase synthesis, in which phthalic acid and potassium hydroxide were chosen as the reactants. The structure and composition of the compound were characterized by chemical and elemental analyses, X-ray powder diffraction, and X-ray crystallography. Low-temperature heat capacities of the compound were measured by a precision automated adiabatic calorimeter over the temperature range from (78 to 400) K. A polynomial equation of the heat capacities as a function of the temperature was fitted by a least-squares method. The smoothed heat capacities and the thermodynamic functions of the compound relative to 298.15 K were calculated and tabulated at an interval of 5 K. In accordance with Hess' law, a thermochemical cycle was designed. The enthalpy change of the reaction of phthalic acid with potassium acetate was determined to be Delta H-r(m)o = -(4.298 +/- 0.493) kJ.mol(-1) by use of an isoperibol solution-reaction calorimeter, and the standard molar enthalpy of formation of C8H5KO4(s) was calculated to be Delta H-f(m)o(C8H5KO4, s) = (1028.05 +/- 0.61) kJ.mol(-1) by use of the enthalpies of dissolution and other auxiliary thermodynamic quantities.