Sputtering of the GaN(0001) surface by Ar+ and N-2(+) ion beams is investigated using synchrotron-radiation photoemission spectroscopy. For Ar+ sputtering, the N atom is preferentially removed and a Ga-enriched GaN surface is produced. The excess Ga atoms on the Ar+-sputtered surface aggregate to form metallic Ga clusters at temperatures above 623 K. A better-ordered GaN(0001)-1 x 1 surface can be obtained by N2+ sputtering, instead of Ar+ sputtering. In addition to acting as a sputtering particle, the N-2(+) ion also serves as a reactant which compensates for the preferential loss of the N atom caused by physical ion bombardment. During chlorination of GaN, chlorine preferentially reacts with surface Ga atoms to form Ga chlorides. Although Ga monochloride (GaCl) is the major product formed on the N-2(+)-sputtered surface, however, volatile chlorides (GaCl2 and GaCl3) are mainly produced on the Ar+-sputtered surface. The formation of volatile products on the Ar+-sputtered GaN surface may result in higher etching rates and lower etching temperatures for ion-assisted chemical etching.