Thionalide loaded acrylic resin (Bio Beads SM-7) was used for separation of As(III) from As(V) and their preconcentration from natural waters. At pH 8.0 As(III) was retained on the column bed while As(V) passed to the effluent. As(V), after reduction to As(III), was then retained on another column from 4 mol l(-1) hydrochloric acid solution. The retained arsenic was eluted with concentrated nitric acid and determined by GF AAS. As(III) and As(V) were determined in samples of river, well and mine waters.