Abstract 1,5-Bis[(2-pyridyl)-3-sulphophenyl methylene] thiocarbonohydrazide (PSTH) immobilized on an anion-exchange resin (Dowex) has been used for the on-line preconcentration of mercury from biological samples and waters prior to its determination by inductively coupled plasma atomic emission spectroscopy. The metal was eluted from the column using a solution of 2 M HNO 3 and mixed on-line with SnCl 2. The optimum experimental conditions were evaluated for the continuous preconcentration of Hg, the direct generation of mercury vapour and the final determination of this element by ICP–AES. The enrichment, together with low blank levels of the optimized procedure, allow the simple determination of this toxic element at concentrations down to a few nanograms per milliliter. The proposed method has a linear calibration range 5–1000 ng ml −1 of mercury, with a detection limit of 4 ng ml −1 ( S/ N=3) and a sampling rate of 40 h −1, investigated with a 9 ml sample volume. The precision of the method (evaluated as the relative standard deviation obtained after analyzing ten series of ten replicates) was ±3.6% at the 10 ng ml −1 level of Hg(II) and ±1.3% at the 100 ng ml −1 level. The accuracy of the method was examined by the analysis of certified reference materials.