Stoichiometric iron-molybdate and an industrial like catalyst, with the usual Mo excess, were prepared by the normal coprecipitation technique. Additionally, a Mo rich catalyst was prepared by sol-gel like technique. The catalytic stability was tested in the presence and in the absence of water in the reactor feed. Only the stoichiometric catalyst deactivates appreciably in the tested conditions. Water in the reactor feed accelerates this process and exhibits a marked reaction inhibition effect. Water seems to hamper the catalyst reoxidation during reaction. The results of the applied characterisation techniques evidenced that the deactivation mechanism involves an increase of surface reduction (higher Fe2+/Fe3+ atomic ratio) and surface Mo loss by MoO3 sublimation and formation of Mo-methanol and Mo-water volatile compounds. Mo species migrate from the bulk to the catalyst surface to compensate the Mo loss favoured by water. Mo excess is in fact required to have a stable, active and selective iron-molybdate catalyst for the methanol to formaldehyde oxidation. (C) 2003 Elsevier Science Ltd. All rights reserved.