A flow injection system is proposed for the rapid and sensitive determination of ultra trace amounts of osmium as osmium tetroxide. The method is based on the catalytic effect of osmium on the oxidation of gallocyanine by hydrogen peroxide at 30°C. The effect of important parameters, such as pH, reagent concentration, and flow rate, sample volume, temperature and length of the reaction coil are reported. Osmium in the range of 0.010 - 100 ng.ml−1can be determined at a rate of 35 ± 5 samples per hour. The limit of detection was 0.003 ng.ml−1. The relative standard deviation for the determination of 1.00 ng.ml−1of Os(VIII) is 1.5%. A system is proposed for elimination of potential interferences. The method was applied to the determination of osmium in real samples.