The loss of molybdenum from industrial iron molybdate (Fe-2(MoO4)(3)) catalyst pellets with an excess of molybdenum oxide was studied during selective oxidation of methanol to formaldehyde for up to about 10 days on stream at varying reaction conditions (MeOH = 1.6-4.5%, O-2 = 2.5-10%, H2O = 0-10.2 vol% in N-2 and temperature = 250, 300 and 350 degrees C). The changing morphology and the local elemental composition in the pellets were followed for increasing time on stream. Molybdenum was shown to volatilize, leaving a depleted zone starting at the pellet surface and moving inwards with time. For temperatures = 300 degrees C only volatilization of the excess MoO3 phase was observed. Increasing concentration of MeOH and temperature enhanced the rate of volatilization, the oxygen concentration had negligible effect, while increasing the H2O concentration decreased the volatilization rate. At 350 degrees C (MeOH = 4.5%, O-2 = 10%, H2O = 0% in N-2) Mo in the Fe-2(MoO4)(3) phase was furthermore volatilized leading to the formation of the reduced ferrous molybdate (FeMoO4). A dynamic 1D mathematical model for a single pellet, in which methanol oxidation to formaldehyde and simultaneous volatilization of free MoO3 takes place, was developed. The model parameters were fitted using experimental data of the pellet weight loss while the evolution of the MoO3 depletion layer thickness was used to validate the model. The model describes the data well and additionally predicts that deposition of MoO3 behind the depletion layer front occurs under certain conditions, leading to a MoO3 deposition layer, which was verified by scanning electron microscopy (SEM) combined with energy-dispersive X-ray spectroscopy (EDS). Simulations with the model show that the overall loss of molybdenum is significantly slower for large pellets compared to small pellets, which is a key parameter for the success of the industrial process.