Buildings represent about 40% of the EU-27 final energy consumption, and 36% of the total CO2 emissions. A major part of these are due to heating and cooling; to maintain comfortable indoor conditions. The EU has committed to meet 20% of all energy demand through the use of sustainable energy sources by 2020. Balancing the energy supply-demand in building air conditioning systems is critical to meet these goals. Thermal energy storage (TES) is a mature technology that has been researched for decades and is a potential solution to sustainably achieve the building requirements in terms of heating/cooling. Thermochemical heat storage (THS) has significant advantages when compared to sensible and latent heat storage, which includes higher storage density, lower volume requirements, low heat loss (approaching zero), and lower charging temperature. In this study, a number of hygroscopic salts and desiccant matrices (salt in matrix, SIM) candidate materials were chosen for open thermal energy storage (TES) systems. The main aim of this study was to characterize different SIM composites to find the best performing composite in terms of low raw material costs, low regeneration temperature for charging, a high temperature lift during reaction, high energy density and high cyclic efficiency. Candidate materials used was vermiculite as host matrix while using CaCl2, MgSO4, LiCl and MgCl2 as salts. The pore geometry and distribution of pore sizes was identified using Mercury intrusion porosimetry (MIP), while, Q600 SDT was used to calculate the hygrothermal properties of the SIMs and Gas Displacement Pycnometry System (AccuPyc) was used to identify the composites densities. Vermiculite with calcium chloride (V-CaCl2) displayed highest potential as a thermal energy storage composite when compared with the other SIMs.