Surface species formed on MoO3/SiO2 catalysts prepared by manually grinding mixtures of both oxides in the presence of water vapour and with molybdenum loadings between 2% and 12% were studied by X-ray diffraction, temperature-programmed reduction and scanning electron microscopy. Polymolybdate species have been identified for molybdenum loadings below 4%, these species condensing to form MoO3-like phases and orthorhombic MoO3, as the weight loading increases.