In this work, we made an attempt to understand the influence of the annealing treatment on CO sensing with tin dioxide based sensors. As a first step, the surface of tin dioxide powders and of sensitive layers based on them has been characterised by means of the IR studies; it was shown in this way that at the surface of thick film SnO2 based sensors the structure of hydroxyl groups is similar to the one found at the surface of SnO2 powders, independent on the sensors' annealing temperature. Afterwards, the CO reaction with sensors annealed at 500 and 700 degrees C has been investigated by simultaneous electrical and infrared studies (DRIFTS) in operando conditions (air, 200-400 degrees C). It was concluded that the final annealing temperature influences the concentration of the reactive sites for oxygen ionosorption, which finally determines the main CO reaction route and thus the sensor signal. (c) 2007 Published by Elsevier B.V.