Composite phase change materials (PCMs) can be used for passive thermal energy storage (TES) applications in buildings under different climatic conditions. From this perspective, two composites of polyethylene glycol(PEG600) with gypsum and natural clay as new kinds of building composite PCMs (BCPCMs) were prepared using vacuum impregnation method. The prepared BCPCMs were characterized by using SEM, FT-IR, DSC and TG analysis techniques. The SEM and FT-IR results proved the existence of good compatibility between the components of the composites due to capillary and surface tension forces. From the DSC analysis, the melting temperatures and the latent heat capacities of PEG600 (18 wt%)/gypsum and PEG600 (22 wt%)/natural clay composites were determined as 10.55 +/- 1 0.07 C, 24.18 +/- 1.0 J/g and 10.85 +/- 0.05 degrees C, 28.79 +/- 1.07 J/g, respectively. Thermal cycling test and TG analysis results exhibited that the prepared BCPCMs have good thermal reliability, chemical and thermal stability. In addition, the thermal performance test demonstrated that compared to the control cubicle, the inside center temperatures of the cubicle made of PEG600/gypsum and PEG600/gypsum wallboards were recorded to be lower as averagely 2.08 +/- 0.02 C for 60 min 1.47 +/- 0.01 C for 120 min, respectively during the heating periods. (C) 2013 Elsevier B.V. All rights reserved.