The phase transition performance of the CaCl (2)center dot 6H(2)O-Ca(NO3)(2)center dot 4H(2)O composite salt system with nucleating and thickening agents was investigated in this paper. The CaCl2 center dot 6H(2)O-Ca(NO3)(2)center dot 4H(2)O composite salt system was prepared by adding Ca(NO3) 2 center dot 4H2O (12 wt%) to CaCl2 center dot 6H(2)O. Different nucleating agents including SrCl2 center dot 6H(2)O, SrCO3, BaCl2, BaCO3, Na2B4O7 center dot 10H(2)O, H3BO3 and NH4Cl were used to address the problems of phase segregation and supercooling phenomena during the phase change process. The results show that the single nucleating agent SrCl2 center dot 6H(2)O or the mixture of nucleating agents with 2 wt% SrCl2 center dot 6H(2)O, 1 wt% BaCl2 and 0.5 wt% of thickening agent carboxyl methyl cellulose is the most suitable for this system. The latent heat remained constant at about 116 J/g before and after adding the agents. Density functional theory was used to further investigate the microstructure-related reason for the salt-water separation and supercooling phenomena. It can be deduced that the hydrogen bond is the vital factor involved during the phase transition. The aim of adding thickener was to form more hydrogen bonds which encapsulated the hydrated species and made it difficult to lose the hydrated waters. The main purpose of adding nucleating agent was to break the metastable state among microscopic species. The results of this work indicate that the CaCl2 center dot 6H(2)O-Ca(NO3)(2)center dot 4H(2)O salt mixture has potential as a thermal energy storage material. Copyright (C) 2017 John Wiley & Sons, Ltd.