An experimental set-up for on-line Hg2+ reduction and determination was devised using the reverse flow injection analysis (r-FIA) concept and the cold vapour (CV) technique, injecting an acidic Sn2+ solution into the mercury sample line. The elemental mercury generated is separated from the reacting mixture by a 100 mi min(-1) helium stream, which passes through a gas-liquid separator connected to a permeation cell. This gas stream is used as the plasma medium. The permeated Hg-o is then concentrated on a 0.3 g gold foil placed inside a quartz tube connected to an 11 W He de discharge plasma chamber. The mercury retained on the gold surface is released by resistive heating and the emission intensity is observed at the 253.7 nm mercury line. For an injection cycle of 30 s, the calibration graphs are linear up to 50 ng mi(-1) (P = 0.999). An injection frequency of 120 h(-1) is achieved, with negligible carry-over. The calculated relative standard deviation of the transient peaks is 1.6%. Higher sensitivities can be achieved using longer injection cycles. Samples of Human Hair Certified Reference Material were used to determine the accuracy of the method.498787795