The high selectivity of iron molybdate catalysts for the production of formaldehyde from methanol is somewhat surprising in view of the very different behaviour of the individual oxides of Fe and Mo for this reaction. The former, on its own, is a complete combustor of methanol, whereas the latter is highly selective. Here we use STEM (scanning transmission electron microscopy) at high resolution to image the surface of small particles of the catalyst and to show that this high selectivity is due to the dominance of the surface region by molybdenum oxide.