The effective use of nanocrystalline semiconductors requires control of the chemical and electrical properties of their surfaces. We describe herein a chemical functionalization procedure to passivate surface states on GaAs nanocrystals. Cl-terminated GaAs nanocrystals have been produced by anisotropic etching of oxide-covered GaAs nanocrystals with 6 M HCl(aq). The Cl-terminated GaAs nanocrystals were then functionalized by reaction with hydrazine or sodium hydrosulfide. X-ray photoelectron spectroscopic measurements revealed that the surfaces of the Cl-, hydrazine-, and sulfide-treated nanocrystals were As-rich, due to significant amounts of As0. However, no As0 was observed in the photoelectron spectra after the hydrazine-terminated nanocrystals were annealed at 350 degrees C under vacuum. After the anneal, the N 1s peak of hydrazine-exposed GaAs nanocrystals shifted to 3.2 eV lower binding energy. This shift was accompanied by the appearance of a Ga 3d peak shifted 1.4 eV from the bulk value, consistent with the hypothesis that a gallium oxynitride capping layer had been formed on the nanocrystals during the annealing process. The band gap photoluminescence (PL) was weak from the Cl- and hydrazine- or sulfide-terminated nanocrystals, but the annealed nanocrystals displayed strongly enhanced band-edge PL, indicating that the surface states of GaAs nanocrystals were effectively passivated by this two-step, wet chemical treatment.