This paper describes a method for the estimation of the thermal expansion in polycrystalline hybrid systems. These new materials have been developed for use in high temperature thermal storage applications. A simple crystallite model is compared with the complex model specified by Hashin and Shapery. After describing and validating the models, the calculated expansions are compared with measurements on the following different hybrid systems : MgO/Na2SO4, Na2SO4-fraction 10-45% SiO2/Na2SO4, Na2SO4-fraction 20-50% The results show that all measurements are within the calculated second order boundaries. A good estimation is also given by the simple crystallite model. The maximum deviation between calculated values and measurements at 800 degrees C is less than 10%. This model can also be applied for SiO2 as a carrier component.