X-ray diffraction and Raman spectroscopy were used in this study to characterize arsenate phases in the arsenate-ferrihydrite sorption system. Evidence has been obtained for surface precipitation of ferric arsenate on synthetic ferrihydrite at acidic pH (3-5) underthe following experimental conditions: sorption density of As/Fe approximately 0.125-0.49 and arsenic equilibrium concentration of <0.02-440 mg/L. Surface precipitation occurred under apparently undersaturated (in the bulk solution phase) conditions, and probably involved initial uptake of arsenate by surface complexation followed by transition to ferric arsenate formation on the surface as indicated by XRD analysis. At basic pH (i.e., pH 8), however, no ferric arsenate was observed in arsenate-ferrihydrite samples at a sorption density of As/Fe approximately 0.125-0.30 and an arsenic equilibrium concentration of 2.0-1100 mg/ L. At pH 8, arsenate is sorbed on ferrihydrite predominantly via surface adsorption, and the XRD patterns resemble basically that of ferrihydrite.