Porous polycrystalline silicon (PPS) based conductivity sensors were fabricated and characterized in this work. The PPS sensors show excellent sensitivity for detecting ambient pressures, gas species as well as temperature. Results show that the current increases significantly as the pressure decreases and, an over two orders of magnitude change has been detected when the pressure was. reduced to 10(-2) atm. We also find that both ethanol and acetone vapors can significantly change the conduction in the sensors. In acetone vapor, the device even displays diode-like characteristics. Electrical conduction mechanisms of the sensors in vacuum and organic vapors are proposed. The fabrication process can be easily integrated with very large scale integrated technology.