With density-functional calculations we have investigated adsorption and diffusion of an Au atom and an Au2 dimer on a θ-Al 2O3(001) surface. The surface structure of θ-Al 2O3(001) has an armchair-like configuration containing flat and trench areas and the Aun (n = 1 or 2) cluster prefers to adsorb on the flat area. A single Au atom adsorbs on an O-Al bridge site with adsorption energy 0.35 eV, whereas an Au2 dimer bonds to the oxide with adsorption energy 0.78 eV, with one Au coordinated singly to a surface O. Formation of Au2 from Au1 is favored, with a negligible energy barrier. The calculated energy barriers for diffusion indicate that an Au atom diffuses more rapidly than an Au2 dimer but both prefer to diffuse anisotropically, along the flat area of the θ-Al2O 3(001) surface.