Atomic force microscopy was used to measure the dimensions of grain-boundary thermal grooves on the surfaces of Al2O3, 100ppm Y-doped Al2O3, and 500ppm Y-doped Al2O3 ceramics heated at temperatures between 1350 degrees C and 1650 degrees C. The measurements were used to estimate the relative grain-boundary energies as a function of temperature. The relative grain-boundary energies of Al2O3 decrease slightly with increased temperature. When the doped samples were heated, there was an overall increase in the grain-boundary energy, attributed to a reduction in the grain boundary excess at higher temperature. The overall trend of increasing grain-boundary energy was interrupted by abrupt reductions in grain-boundary energy between 1450 degrees C and 1550 degrees C. In the same temperature range, there is an abrupt increase in the grain-boundary mobility that is associated with a complexion transition. When the 100ppm Y-doped sample was cooled, there was a corresponding increase in the relative grain-boundary energy at the same complexion transition temperature, indicating that the transition is reversible.