A study was undertaken to ascertain the analytical capabilities of combined HPLC-ICP-AES with ultrasonic nebulization or HPLC-ICP-MS systems to perform on-line separation of Cr "'-Cr"' and on-line preconcentration of Cr"'. Tetrabutylammonium bromide was used as the ion-pair forming agent and a reverse phase C,, column was used for separation. By optimizing the conditions of separation and preconcenhation as well as plasma parameters a method could be developed for the determination of Cr"' and Cr"' in seawater by HPLC-ICP-AES with ultrasonic nebulization. In this matrix detection limits (3a criterion) turned out to be 4.6 ng/ml for Cr"' and 3.7 ng/ml for Cr"', while the RSD is 2%-3% at the level of 100 ng/ml Cr. As the preconcentration of Cr "' is regarded, the detection limit of ICP-AES depends, among others, on the sample volume loaded onto the column. It resulted to be 0.20 ng/ml for an initial volume of 2 ml, while the RSD at 10 ng/ml CrW was +_3.8%. On the other hand, the high salinity of seawater and C content of the separation eluent pose serious problems with ICP-MS detection. In fact, in this matrix only Cr"' preconcentration was achieved with a detection limit of 0.12 ng/ml for Cr"' (initial volume 2 ml). Due to isobaric interferences, the isotope at mass 50 (relative abundance 4.31%) was preferred.