This work presents the effects of ambient conditions, in particular oxygen and humidity, on the dielectric spectra of thin zinc phthalocyanine (ZnPc) films equipped with interdigitated electrodes and the effect of annealing in dry N-2 or in ambient air. The measurements were performed in the frequency range 10(-2)-10(5) Hz. The results indicate that the electric properties of ZnPc films are not only affected by oxygen but also by water vapour the presence of which always leads to the drop in alternating current conductance (ac-conductance). Moreover, at room temperature, the ac-conductance of ZnPc films previously exposed to air exhibits a reversible change with humidity, which makes these films attractive for humidity sensing applications. (C) 2004 Elsevier B.V. All rights reserved.