A simple and rapid method is described for reliable determination of ultra-trace concentrations of mercury based on a novel combination of a sequential injection (SI) system with cold vapour atomic absorption spectrometry (CVAAS). The SI-CVAAS method achieved a very high sampling frequency of 180 h-1, a detection limit of 0.2-0.3 ng mL-1, a dynamic range of 0-60 ng mL-1 and requires only 0.2-0.6 mL of sample. The presence of common metal ions, such as Cu2+, Pb2+, Cd2+, Zn2+, Fe3+, Co2+, Ni2+ and Mn2+, did not interfere with the measurement of mercury by this method. Also the interference from organic matter, such as humic substances, was easily reduced or eliminated by appropriate sample dilution. The method was successfully applied to the determination of mercury in lake sediment samples and a river sediment reference material. An average percentage recovery of 101.2% was achieved by the SI-CVAAS method for mercury in the reference material with a R.S.D. of 1.8%.