An automated on-line solvent extraction system has been developed for the determination of arsenic in steels by electrothermal atomic absorption spectrometry (ET-AAS). It is based on the reaction of As(III) with iodide ion in the concentrated hydrochloric acid medium to produce AsI₃, which is extracted into benzene and back-extracted into water. Improved gravitational phase separator based on the previously proposed was developed for the recycling of organic solvent used in the automated on-line solvent extraction system. Using the proposed automated on-line solvent extraction system, arsenic contained in the acid decomposed steel sample solution was automatically extracted into the benzene phase and it was back-extracted into the water phase. Then, the back-extracted water phase was used for the determination of arsenic by ET-AAS. When the ET-AAS method was used for the determination of arsenic, 800 mg dm^-3 of cobalt solution had to be used as the matrix modifier to remove an effect of coexisted substances such as iodide ion. In this method, a determination limit of As is 0.2μg in the 0.1g of steel sample.