The temperature-composition phase diagram of the butyronitrile-chloroethane system was determined by differential thermal analysis. The butyronitrile-chloroethane system is a simple eutectic, the eutectic point being -185 degrees C (88 K) and 48 mol % butyronitrile. The thermodynamics of the liquid phase were tested against a number of solution models and were found to be best represented by the associated solution model with the solution consisting of free butyronitrile, free chloroethane, and dimolecular complexes composed of one butyronitrile and one chloroethane. This model was also used to estimate the vapor pressure of butyronitrile-chloroethane solutions.