STM data show that gold exhibits homogeneous nucleation and growth on polycrystalline titania surfaces. When very small Au particles are exposed to ambient air at room-temperature rapid sintering occurs (similar to20 nm) accompanied by Au migration to titania crystallite grain boundaries. This process is driven principally by oxygen. Prolonged exposure to air induces further Au particle mobility resulting in agglomeration of the similar to20 nm particles at titania crystallite boundaries. It appears that water adsorption plays a role in this process. Work function measurements indicate that the very small Au clusters acquire a net positive charge and photoemission results reveal that their Au 4f BEs are increased relative to bulk gold. They also exhibit strongly enhanced adsorption and decomposition of styrene. This chemical effect is not a consequence of the charge state of the Au particles, rather it is a manifestation of a quantum size effect. The results are of significance to an understanding of the properties of Au/titania catalysts.