A proper storage temperature is an important criterion for selecting a phase change material (PCM) for a passive solar heating application. Here we describe a novel procedure to produce a mixture of carboxylic acids with a melting temperature adjustable to the climate specific requirements. The approach is based on the ideal solution model and differential scanning calorimetry (DSC). The applicability of the method is demonstrated and it is also applied to a PCM wall design. The accuracy of the theoretical model is +/- 2-degrees-C in the temperature range of 20-degrees-30-degrees-C and even a +/- 0.5-degrees-C accuracy can be obtained by the experimental procedure.