Abstract The polishing removal rate and surface quality of sapphire (Al2O3) varies greatly with crystal orientation when chemical effects couple with abrasive removal in chemical mechanical polishing (CMP). The relationship of orientation, solution chemistry and abrasive were studied for sapphire with c (0 0 0 1), a ( 1 1 2 0 ), and m ( 1 0 1 0 ) orientations. Aqueous abrasive slurries of alumina, monocrystalline diamond, and polycrystalline diamond were compared at various pH’s. Orientation-dependent removal rate as determined by weight loss was greatest for the c (0 0 0 1) orientation, with a remarkably high removal rate of 1.0 mg/h for alumina slurry at pH 12. Surface quality was characterized with atomic force microscopy (AFM) in terms of RMS roughness and scratch depth. The optimum CMP removal by alumina also yielded a superior surface finish of 0.3 nm RMS roughness. Results are examined in light of atomic structure and hydration layer formation. It is proposed that the α-alumina abrasives experience surface hydration similar to sapphire, and a chemical–mechanical reaction between the sapphire and the α-alumina abrasive hydration layers promotes accelerated material removal.