A new method for the determination of low concentrations (0.03-35 microM) of the aqueous ferrate (Fe(VI)) was developed. The method is based on the reaction of Fe(VI) with 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonate) (ABTS) which forms a green radical cation (ABTS(+)) that can be measured spectrophotometrically at 415 nm (ABTS method). The reaction of Fe(VI) with ABTS has a stoichiometry of 1 : 1 in excess of ABTS (73 microM). The increase in absorbance at 415 nm for ABTS*+ generation was linear with respect to Fe(VI) added (0.03-35 microM) in buffered solutions (acetate/phosphate buffer at pH = 4.3) and was (3.40+/-0.05) x 10(4) M(-1) cm(-1). The reaction of Fe(VI) with ABTS was very rapid with a half-life time below 0.01 s at pH 4.3 and 73 microM of ABTS. This enables the ABTS method to measure Fe(VI) selectively. The residual absorbance of ABTS*+ was found to be stable in several water matrices (synthetic buffer solution and natural waters) and concentrations of Fe(VI) spiked in natural waters could be determined with high accuracy. The ABTS method can also be used as a tool to determine rate constants of reactions of Fe(VI). The second-order rate constant for the reaction of phenol with Fe(VI) was determined to be 90 M(-1) s(-1) at pH 7.