A quartz reaction vessel is designed and constructed to interface HPLC and cold-vapor AA for mercury speciation in aqueous samples. Inorganic mercury, methyl mercuric and ethyl mercuric compounds can be separated effectively within 10 minutes. An on-line preconcentration unit is used to reduce the limit of detection (LOD) before the sample is introduced into the reversed-phase HPLC. Without preconcentration, the detection limits for the three mercury species are 0.094, 0.085 and 0.124 ppm, respectively. Limits of detection are reduced to 0.78, 0.78 and 0.42 ppm, with the use of preconcentration technique. The feasibility of this method is tested by analyzing mercury compounds in synthetic tap and river water samples.