A surface-modified silica gel has been developed for the pre-concentration of arsenite from natural waters. The reaction of silica gel with (3-mercaptopropyl)trimethoxysilane produces a mercapto-modified silica gel with a silver capacity of ca. 0.9 mmol g–1. This material selectively removes arsenite from samples that also contain arsenate, monomethylarsonate and dimethylarsinate, and can be employed for both the column and batch pre-concentration of arsenite. Under column and batch conditions recoveries of ca. 96 and 94% are obtained but reproducibility is better for the column technique.