Phase-change thermal-storage materials (PCMs) can be used in many fields, such as building heating and solar energy utilisation and so on. Although the main problems with inorganic PCMs – for instance, phase separation and supercooling – have been studied for many years and solved to a certain degree, the research has been mainly concerned with analytical reagents or chemical pure-grade materials. The problem for industrial-grade materials has yet to be solved. In this paper, the performance of industrial-grade disodium hydrogen phosphate dodecahydrate as a PCM is improved by the addition of sodium sulfate decahydrate, graphite and sodium silicate. The results show that the heat-storage performance of the composite hydrated salt Na₂HPO₄·12H₂O–Na₂SO₄·10H₂O with a ratio of 9·5:0·5 and the addition of 6% graphite and sodium silicate (7:5) is much better than those of Na₂HPO₄·12H₂O and Na₂SO₄·10H₂O materials alone. The supercooling degree of the composite hydrated salt is reduced to about 0·8°C without phase separation. In addition, it exhibits stable phase-transition characteristics and good heating–cooling cycling behaviour in the range 20∼70°C with no phase separation and less supercooling after 30 cycles.