The enthalpy of solution of terfenadine in ethanol/water mixtures, 0-20.5 wt.% of water was determined by calorimetry. A Sudden increase of 1-2 kJ mol(-1) in the enthalpy at a concentration value around 0.01 mol kg(-1) is observed. This step in the enthalpy is interpreted as due to solute n-mer aggregates formation. The solubility of terfenadine in the cosolvent systems used in the calorimetric studies was determined.