In this article, we present new experimental data regarding the influence of ammonia (NH3) and water (from wet atmospheres) in the conducting properties of lutetium bisphthalocyanine (LuPC2)-based films in two very different structural features, namely Langmuir-Blodgett (LB) and vacuum evaporated (VE) films, deposited onto interdigitated electrodes. We pay particular attention to the effect of the mass flow rate ratios of the active gases, which certainly influence the mechanism of conduction of the chemiresistors. The particular trends observed are discussed on the basis of two main contributions: the electronic effects and the competition between gases in the adsorption process. (C) 2007 Elsevier B.V All rights reserved.