  We report the effect of air exposure on the electronic properties of
pentacene single crystals. Air can diffuse reversibly in and out of the
crystals and controls the physical properties. We discern two competing
mechanisms that modulate the electronic transport. The presence of oxygen
increases the hole conduction, as in dark four O2 molecules introduce one
charge carrier. This effect is enhanced by the presence of visible light.
Contrarily, water, present in ambient air, is incorporated in the crystal
lattice and forms trapping sites for injected charges.
