The construction and calibration of an adiabatic calorimeter for samples of about 0.4 g is described. The calorimeter works between 10 and 420 K, the accuracy is 0.5% for heat capacity measurements and 0.2% for latent energy effect like the enthalpy of fusion. The solid-liquid phase diagram of the system naphthalene-durene is presented. The phase diagram has been determined by three measurements, the interrupted way of measuring in adiabatic calorimetry was used to calculate the liquid composition at different enthalpy levels. The phase diagram is an example of an almost ideal eutectic system. The excess Gibbs energy of the liquid phase at 360 K was found to be in the order of 100 J mol(-1) for the equimolar mixture.