Porous silicon (p type) has been exposed to several chemical vapors at various partial pressures. The quenching of the photoluminescence by the adsorbates has been quantified and correlation to the electrical conductivity of the porous silicon sample has been studied. Some gases, e.g., water and benzene, have a small effect on the photoluminescence and on the conductivity, while others, e.g., methanol, reduce the photoluminescence by a factor of 2 and increase the conductivity by four orders of magnitude. This is accompanied with a qualitative change in the current-voltage characteristics. These changes have been found to be reversible and the temporal behavior of the system has been investigated.