Abstract: A rapid, sensitive and automatic method for determination of differential valence state of Fe²⁺ and Fe³⁺ after on-line preconcentration of Fe²⁺ in a RP-C₁₈ microcolumn using 1,10 phenanthroline as a complexing agent with FAAS has been developed. The absorbed Fe²⁺ phen complex in the microcolumn has been eluted with ethanol into the nebulizer of FAAS, and ascorbic acid for the reduction of Fe³⁺ to Fe²⁺ has been used. Various parameters affecting complex formation and the subsequent complex absorption in the microcolumn were optimized. A good precision (%RSD=1.1 for n=10), high enrichment factor(19) and sample throughput (80/h) with detection limit 3 μg/L were obtained. The method has been successfully applied on the synthetic water samples for the valence state determination of iron with the recovery 94.5%～103.6%。