The conductance of amorphous Si films produced by glow discharge of SiH4is found to be very sensitive to various adsorbates such as water, ammonia, and dimethyl ether. Films exposed to air and light must first be heated to 150 °C in vacuum to remove adsorbed moisture and the reversible photoelectronic effect discovered by Staebler and Wronski. When exposed to adsorbates, the conductance of undoped films (about 0.36 &mgr;m thick) decreases by several orders of magnitude; the conductance of lightly phosphorus‐doped films first increases rapidly and then slowly decreases to a saturation value which lies above the conductance of the annealed films. Heating in vacuum at 150 °C restores the original values. The results indicate that the conductance changes produced by light and by adsorbates are two separate processes, the latter being a surface effect.