In this article the analysis of steady state and transient gas sensing characteristics of undoped and Pd surface doped SnO2 films, deposited by spray pyrolysis, is described. The influence of parameters such as air humidity (2-50% RH), operation temperature (25-500 degreesC) and Pd surface concentration (0-1% ML Pd) on gas response to CO and H-2 (0.1-0.5%), response time, shape of sensitivity S(T) curves and activation energy of tau(1/kT) dependencies are discussed. A mechanism based on a chemisorption model is proposed to explain how Pd influences the gas sensing characteristics of SnO2 films. (C) 2003 Elsevier Science B.V. All rights reserved.