We report on the interactions between spin-coated pentacene films and molecules present in air (nitrogen, oxygen, and moisture) studied by means of micro-Raman spectroscopy. We found that while oxygen does not interact with pentacene, water molecules diffuse in the film proportionally to the relative humidity of the ambient, giving rise to an interaction that affects pentacene's optical properties. These findings are fully reversible when dry ambient is restored, and they could be ascribed to the formation of a reversible water-pentacene complex theoretically calculated. This interaction affects a specific pentacene vibration mode that could lead to reversible formation of electron traps involved in the degradation of the electrical performance of pentacene. (c) 2008 Published by Elsevier B. V.