Chlorobenzene and bromobenzene, liquid substances at room temperature, crystallize with very similar structures, without any known polymorphism at ordinary pressures. They are isomorphous, showing complete solid state solubility, with little deviation from ideality. Thus, as it is confirmed by thermal cycling experiments, their alloys would be suitable as phase change materials for cold storage applications if one needs precise temperature control at a temperature between -31 and -45 degrees C.