Composite silicon-polycrystalline chemical vapour deposition (CVD) diamond wafers are potential substrates for GaN-based devices for use in harsh environments due to their high thermal conductivity and chemical stability. When cooled from a typical diamond deposition temperature of approximately 800 to 25 degrees C wafer bowing arises from a mismatch in the coefficients of thermal expansion of silicon and polycrystalline diamond. In this paper 100 mm diameter silicon-polycrystalline diamond wafers have been modelled using ANSYS finite element software to investigate their bowing behaviour as a function of temperature and geometry. The maximum bow of a wafer occurred where the thicknesses of both the silicon and polycrystalline diamond layers was almost identical; this has been confirmed using analytical methods. Strategies are discussed for reducing wafer bow.