The melting behaviour of binary mixed crystalline materials was measured with adiabatic calorimetry. The experimental results (heat capacity against temperature graphs) are explained in terms of a model that is based on the assumption that the inhomogeneity of the materials can be described by a Gaussian distribution function. Although this model, called ULTRACAL, has only one adjustable parameter, it is capable of reproducing the experimental data with a good degree of accuracy, as demonstrated for various samples of (1,4-dichlorobenzene + 1,4-dibromobenzene). ULTRACAL thus provides a link between the adiabatic calorimetry data and the solid-liquid Tx phase diagram. It is shown that the model can be used for die determination of phase diagrams. By using ULTRACAL to assess the heat capacity against temperature graphs of five samples, the solid-liquid Tx phase diagram of (1,3,5-trichlorobenzene + 1,3,5-tribromobenzene) was determined.