A?sequential?injection?system?for?on-line?sorbent extraction?preconcentration?in electrothermal atomic absorption spectroscopy was developed for the determination of trace thallium in geochemical samples. The TlBr4-1 complex was adsorbed on?a?20-mu L micro-column?(located at the tip of the furnace sampling probe) packed with XAD-8 resin. After sequentially aspirating separate zones of acetone, rinsing acid, and sample (pretreated with bromine) into?a?2.5-m long, 1-mm i.d. holding coil, the flow was reversed and directed to the?column. Sample loading, analyte adsorption,?column?rinsing and analyte elution were achieved within?a?single reversed syringe stroke. The adsorbed analyte was eluted into the furnace with 50 mu L acetone. Mutual mixing between sample, rinsing acid, and eluent were prevented by separating the zones with small air segments during metering. Tightening of?column?packing was avoided by?a?slight back-suction through the?column?after each operational cycle. With 1-mL sample loading, an enrichment factor of 15 was obtained with?a?detection limit of 18 ng/L thallium (3 sigma).?A?precision of 2.4% RSD (n=11, 4 μ g/L) and?a?sampling frequency of 11/hour were achieved. The method was applied to the analysis of geochemical samples. The results were in good agreement with the certified values of standard reference geochemical materials.