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We have studied the effect of substrate material related to thermal mismatch for silicon carbide (SiC) diaphragm-based capacitive pressure sensors. Two sets of devices, with identical dimensions and fabrication processes were made on poly-SiC and Si substrates. Designed for a maximum pressure of 4.83 MPa (700 psi), these devices were operated in small-deflection mode and tested at room temperature and 500°C. At room temperature, the SiC- and Si-substrate devices showed sensitivities of 6.5E-04 and 6.1E-04 fF/Pa, nonlinearities of 5.0% and 3.9%, and resolutions of 0.2% and 0.3%, respectively. At 500°C, the SiC- and Si-substrate devices showed sensitivities of 1.2E-02 and 1.1E-02 fF/Pa, nonlinearities of 2.6% and 3.8%, and resolutions of 0.5% and 1.8%, respectively. For the chosen design parameters, the results show little influence of substrate material on sensor performance.