This paper reports a method developed for the simultaneous determination of methylmercury (MeHg+) and mercury(II) (Hg2+) species in water by liquid chromatography coupled to online UV irradiation and cold vapour atomic fluorescence spectrometry (LC-UV-CV-AFS) after online short-column preconcentration. This work focused on systematic studies of several variables to establish the maximum species recoveries, preconcentration factors and good reproducibility. The optimum results obtained were the following: 0.07 mmol L 1 2-mercaptoethanol as a complexing agent, precolumn conditioning with the mobile phase: a mixture of 80% of methanol (MeOH) and 20% of the following buffer: 0.0015 mol L 1 ammonium pyrrolidine dithiocarbamate (APDC) and 0.01 mol L 1 ammonium acetate (NH4CH3COO) at pH 5.5, 2 cm precolumn length and 2 mL min 1 sample flow. This method was applied to three water samples with different mineralisation contents. Various tests, based on spikes, were performed on each sample. A breakthrough volume of 4 mL was found. The recovery values of 72 3% and 81 5% for MeHg+ and Hg2+, respectively, were obtained regardless of the matrix composition, and the PF values were 30 and 32 for MeHg+ and Hg2+, respectively. The accuracy of the preconcentration method was verified by analysing a certified reference material. The detection limits (LDs) obtained were 15 ng L 1 for MeHg+ and 2 ng L 1 for Hg2+. The quantification limits (LQs) were 50 ng L 1 for both species. The established analytical online preconcentration method is suitable for the quantification of mercury species in a wide range of environmental waters.