The segregation behaviour of a cation (yttrium) with a low solubility in the polycrystalline oxide host (α-Al2O3) has been investigated at temperatures between 1450 and 1650°C using analytical scanning transmission electron microscopy. Three distinct segregation regimes were identified. In the first, the yttrium adsorbs to all grain boundaries with a high partitioning coefficient, and this can be modelled using a simple McLean-Langmuir type absorption isotherm. In the second, a noticeable deviation from this isotherm is observed and the grain boundary excess reaches a maximum of 9 Y-cat/nm2 and precipitates of a second phase (yttrium aluminate garnet, YAG) start to form. In the third regime, the grain boundary excess of the cation settles down to a value of 6–7 Y-cat/nm2 that is in equilibrium with the YAG precipitates. In a material (accidentally) co-doped with Zr, the Zr seems to behave in a similar way to the Y and the Y + Zr grain boundary excess behaves in the same way as the Y grain boundary excess in the pure Y-doped system. In this latter system, Y-stabilised cubic zirconia is precipitated in addition to YAG at higher Y + Zr concentrations.