H-X-T (enthalpy-concentration-temperature) diagram of the working fluid is essential for the optimum design of absorption heat pumps and chillers. One typical procedure is to use the differential heat of dilution and solution between refrigerant and absorbent and heat capacity of the working fluid. The other procedure is to use the vapor pressure and the heat capacity of the working fluid when only one volatile component is contained. However, for the lithium bromide-based working fluid system containing organic component as absorbent, it is hard to measure the differential heat of solution because the absorbent is a mixture of salt and organic. The procedure using vapor pressure must have a certain level of deviation from the true values because another volatile organic component is contained in the working fluid. In this study the widely applicable and accurate method for constructing the H-T-X diagram is newly proposed in order to overcome the limitations appearing internally in the typical procedure. This method needs the differential heat of dilution and the heat capacity of the working fluid. For a newly proposed working fluid system, the differential heat of dilution and the heat capacity were measured using an isoperibol solution calorimeter (CSC 4300).