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Several piezoresistive microelectromechanical (MEMS) sensors are operated in a gamma ray environment to doses of 800 kGy. The pressure transducers and accelerometers are micromachined silicon-on-insulator (SOI) and bulk silicon devices, respectively. Both sensor types experienced similar performance degradation: a drift in offset voltage with a slight increase in sensitivity. We explain the drift in offset voltage for all sensors tested by correlating the change in resistance of the silicon piezoresistors to the formation of oxide and interface trapped hole charges. We demonstrate how these charges effectively reduce the volume for current flow through the piezoresistors due to the creation of a depletion region surrounding the periphery of the gage resistors. Differences in the magnitude of the output voltage drift of the two sensor types are determined to be related to the unique construction of each sensor.