Iron molybdate/molybdenum oxide catalysts with varying content of Mo (Mo/Fe=1.6 and 2.0) were synthesized by a mild hydrothermal method and structurally characterized by XRD, XPS, Raman spectroscopy, SEM-EDX, BET and ICP-OES. The stability of the prepared catalysts in selective oxidation of methanol to formaldehyde was investigated by catalytic activity measurements for up to 100 h on stream in a laboratory fixed-bed reactor (5% MeOH, 10% O-2 in N-2, temp.=380-407 degrees C). Excess MoO3 present in the catalyst volatilized under reaction conditions, which lead to an initial loss of activity. Interestingly, the structure of the excess MoO3 significantly affected the stability of the catalyst. By using low temperature hydrothermal synthesis, catalysts with the thermodynamically metastable hexagonal h-MoO3 phase was synthesized, which yielded relatively large crystals (2-10 mu m), with correspondingly low surface area to volume ratio. The rate of volatilization of MoO3 from these crystals was comparatively low, which stabilized the catalysts. It was furthermore shown that heat-treatment of a spent catalyst, subject to significant depletion of MoO3, reactivated the catalyst, likely due to migration of Mo from the bulk of the iron molybdate crystals to the surface region. Fe-2(MoO4)(3)/MoO3 catalysts for selective oxidation of methanol were synthesized by hydrothermal synthesis forming large hexagonal-MoO3 crystals. Significantly lower rate of catalyst deactivation due to volatilization of MoO3 under reaction conditions was observed for the large h-MoO3 compared to smaller crystals of thermodynamically stable alpha -MoO3.