When partially sintered cubic ZrO2–10 mol% Y2O3specimens are heat‐treated at 1500°C with powder mixtures of MgO and ZrO2–10Y2O3at varying ratios, the grain boundaries migrate, leaving behind new solid solutions enriched with MgO but depleted of Y2O3. With increasing MgO content in the solute source powder, the average migration velocity increases, and the MgO content increases and the Y2O3content decreases slightly in these new solid solutions. With increasing MgO content in the solute source, the grain boundaries tend to be corrugated and faceted. Migration reversal is also observed at the corrugated boundaries. These variations of the grain boundary migration behavior with the MgO content in the solute source are consistent with the diffusional coherency strain energy as the driving force for the mig