Sealed, vacuum cavities are highly useful in silicon-based microelectromechanical systems. Sealed polysilicon cavities serve as reference chambers for commercially available pressure sensors and provide enclosures for high-e mechanical resonators. They also provide isolation of resonant strain gauges from ambient gases and liquids for use in precision pressure transmitters. The residual pressure in such cavities is less than 10(-5) Ton as evidenced by the polysilicon resonator Q: values which can be as high as 300000. Measured values of Q can be dominated by parasitic electrical resistance losses in the electrical drive circuit. After these losses are accounted for, the dominant loss is due to mechanical radiation of acoustic waves from the supports to the rest of the die,and package. The vibration of the beams can be excited and sensed in-several different Bays for testing purposes. Fiberoptically driven and sensed microbeams operating at up to 510 degrees C: for several hours resulted in no loss of vacuum as evidenced by the restoration of resonator Q upon its return to room temperature. Thus optically resonant microbeams are-potentially suitable for fiberoptic-based sensors that withstand harsh environments, including high temperature.