X-ray photoelectron spectroscopy was used to study the surface alteration of olivines (fayalite and forsterite) during acid dissolution (0.05 mol 1-1 H2SO4 at room temperature for one day). The abundances of Fe and Mg, relative to Si, in the near surface of olivines decreased after acid dissolution. The divalent cations in the fayalite were removed more readily than those in the forsterite. After acid dissolution, the Si 2s and O 1s spectra of fayalite were deconvoluted to contributions from unleached and leached phases. The Si 2s and O 1s binding energies were higher for the leached phase relative to the unleached phase, and comparable to those of silicon dioxide (quartz and silica gel), indicating that SiO2 · nH2O formed on the mineral surface. Scanning electron micrographs of the leached olivine grains showed a harshly etched structure for fayalite and a locally etched structure for forsterite, indicating that the dissolution proceeded heterogeneously.