Methods for the on-line preconcentration of Cd²⁺, Cr³⁺, Cu²⁺ and Pb²⁺ and chemical speciation of Cr³⁺ and CrO₄²⁻ using a microcolumn (50 × 7 mm id) packed with about 0.34 g dealginated seaweed biomass are described. Preliminary batch experiments showed that metal sorption by the biomass is rapid, about 90% of the metals in solution is taken up in less than 5 min, and occurs in a wide pH range, 2–7. Maximum binding was obtained between pH 6 and 7 and the effective binding capacities at pH 6, determined from column breakthrough measurements using a flow rate of 0.8 mL min⁻¹, were 4.0, 8.7, 9.4 and 8.5 µmol g⁻¹ for Cd, Cr Cu and Pb, respectively. Unretained CrO₄²⁻ was separated from Cr³⁺ thus making it possible to determine the levels of both species in solution. The analytical procedures developed for metal preconcentration and chromium speciation were validated by analyses of two Lake Ontario water reference materials, TMDA 51.2 and TMDA 54.2, and a synthetic seawater sample. Comparison of the results obtained by the proposed methods with the certified total values using the Student t test at 95% confidence limit showed that the differences were not statistically significant. Independent confirmation of the accuracy of the chromium speciation results was obtained using the measurement of absorbance of the 1,5-diphenylcarbohydrazide–chromium complex, before and after treatment with nitric acid, to determine the levels of CrO₄²⁻ and Cr³⁺ by difference. The results obtained with the proposed method for the analyses of TMDA 51.2 and TMDA 54.2 for Cr³⁺ and CrO₄²⁻ were 20 ± 0.2 and 39 ± 0.3 µg L⁻¹, and 162 ± 1 and 299 ± 1 µg L⁻¹, compared with 12 ± 5 and 54 ± 31, and 132 ± 12 and 313 ± 16 µg L⁻¹, respectively.