This proposal seeks to study three systems associated with heat-storage applications using in situ neutron powder diffraction. The first experiment will focus on a sample of Al-Si alloy embedded in an alumina matrix. The experiment will provide information about the chemical and mechanical behaviour of this composite under conditions of heat cycling. The second experiment will focus on characterising a new hydrate of strontium hydroxide and will use neutron diffraction to locate the hydrogen positions. The third experiment will focus on the stepwise dehydration of magnesium sulfate heptahydrate. The results of these studies will provide valuable information about the mechanisms of operation of these heat-storage materials, and will identify at an early stage in the research programme any potential issues (chemical and mechanical) associated with the use of these materials.