Studying gas sensing effects in n-In2O3 films, We have observed the anomalous behavior of reducing gases such as CO and H-2. At Toper < 250-300 degreesC the decrease of In2O3 film conductivity took place during the interactions with CO and H,. Such effect is specific for n-In2O3 interactions with oxidizing gases. Acceptor-like behavior of conductivity has been observed in humid atmosphere only. The In2O3 films were deposited by spray pyrolysis from InC1(3)-water solutions. Their thickness was varied between 20 and 400 nm. The maximum of acceptor effect took place for In2O3 films with minimal thickness and minimal grain size. The mechanism of CO and H-2 interactions with chemisorbed molecular oxygen has been proposed to explain of the observed effect. According to this model, the low temperature CO and H, oxidation by molecular oxygen is followed by accumulation of atomic oxygen forms (O) on the surface of In2O3. Water plays a role of catalyst in these reactions. Mechanism of such influence is connected with formation of intermediate complexes on the surface of In2O3. (C) 2003 Elsevier B.V. All rights reserved.