To measure enthalpy densities, respectively overall specific heats, frequently the differential scanning thermal analysisis (DSC) is applied. Other useful methods are also available, but all of them have disadvantages. Therefore, an alternative measurement procedure - the online enthalpy measurment method - which was presented six years ago, has now been further improved. The quality of the results can be essentially increased by evaluating the experimental data according to a specially developed method. It is analogous to a method of numerical mathematics. The new evaluation procedure is presented and applied to measurements of the enthalpy density of two different phase change slurries, an ice slurry and a water suspension containing micro-encapsulated paraffin wax.