A procedure for the photometric on-line determination of trace arsenic in aqueous solutions was developed. The procedure is based on the reaction gas extraction of arsenic as arsine followed by liquid-absorption separation from a gas phase into a solution of a photometric reagent (2-(p-nitrophenyl)-3,5- diphenyltetrazolium chloride). The determination limit was 10 μg/L at a sample volume of 50 mL.