A new phase transition microextraction method was developed for determination of trace amount of arsenic ions in water samples in this work. The method is based on the critical point of miscibility of solvents. In this method the mixed solution of sample and organic solvent is initially homogeneous but is separated into two phases by adding a secondary solvent (modifier). In acidic medium As(V) was complexed with ammonium molybdate, this complex was quantitatively extracted to the mixture of organic solvent (acetonitrile/methyl isobutyl ketone) before ETAAS determination. Total inorganic arsenic (III, V) was extracted similarly after oxidation of As(III) to As(V) with nitric acid. Concentration of As(III) was calculated by difference in the concentration between total arsenic and As(V). Optimization of the experimental conditions and instrumental parameters was investigated in detail. A detection limit of 0.05 mu gL(-1) with enrichment factor of 85 was achieved for only 5 mL of sample. The analytical curve was linear in the concentration range of 0.25-4.00 mu gL(-1). Relative standard deviation (RSD) for 10 replicate determinations of 2.0 mu gL(-1) of As(V) was 4,1%. The method was successfully applied to preconcentration and determination of arsenic in real water samples.