AsI3is demonstrated to partition from a 9 M HC1, 0.10 M KI, 0.10 H ascorbic acid solution into kerosene and dodecane membranes which do not contain complexing agents. Subsequent hydrolysis at the membrane receiver interface causes a transport of As against its concentration gradient. A 60 minute experiment with a 4 mL receiver solution and a 200 mL sample permits the preconcentration of As by a factor of 16 ± 0.5. The interferences of aluminum and phosphate on the graphite furnace atomic absorption spectro-metric determination of As are eliminated. This transport mechanism, which has not been previously reported, may allow the range of species which can be preconcentrated by transport across supported liquid membranes to be significantly extended.