A new method for determining ultra-trace levels of hexavalent chromium in ambient air has been developed. The method involves a 24-h sampling of air into potassium hydroxide solution, followed by silica gel column separation of chromium (VI), then preconcentration by complexation and solvent extraction. The chromium (VI) complex was dissolved in nitric acid. The resultant chromium ions were determined by inductively coupled plasma mass spectrometry (ICP-MS) using a dynamic reaction cell (DRC) with ammonia as the reactive gas to reduce polyatomic interferences. The interconversion of chromium in potassium hydroxide solution and air sample matrix were investigated under ambient conditions. It was found that there was no conversion of chromium (VI) into chromium (III) species. However, it was observed that some chromium (III) species were converted into chromium (VI) species. For a KOH solution containing 100 mug l(-1) of chromium (III) species, the rate of conversion was found to be 3% after 24 h exposure, 8% after 48 h, 10% after 72 h and no further conversion was observed thereafter. However, in a solution containing air sample matrix, 9.3% of chromium (III) converted to chromium (VI) within 6 h, and during the course of a 11-day exposure period, 13% (range 8-17%) of chromium (III) converted to chromium (VI). The method detection limit (MDL) for chromium (VI) in potassium hydroxide solution (0.025 M) was found to be 2x10(-2) mug l(-1). This is equivalent to 0.2 ng m(-3) (for 23 m(3) air sampled into 200 ml of KOH solution over a 24-h period). The recovery of spiked chromium (VI) from solutions containing air sample matrix was 95+/-9% (n=8). Matrix related interferences were estimated to be less than 10% based on recovery studies. The concentration of airborne chromium (VI) in Sydney residential areas was found to be less than 0.2 ng m(-3), however, in industrial areas the concentrations ranged from 0.2 to 1.3 ng m(-3) using this analytical procedure.