Metal speciation was carried out by on-line hyphenation of capillary electrophoresis (CE) with mass spectrometry (MS) via an electrospray ionization (ESI) interface. The commercially available interface was hardly able to produce stable electrospray conditions over an extended period of time, mainly caused by an insufficient positioning of the CE capillary inside the ESI stainless steel tip. A device was developed, which allowed an infinitely variable adjustment of the capillary. The optimum position for stable electrospray conditions was set to 0.4-0.7 mm outside the ESI tip. Off-line ESI-MS investigations of free metal ions [Cu(II)], metal ion-containing complexes [CuEDTA, (CH 3 ) 3 SbCl 2 ] and covalent organometallic compounds (selenocystamine, selenomethionine) were carried out in order to assess the suitability of the technique for metal speciation. The usefulness of ESI-MS as a detection method largely depends on the stability of the analysed species. Inorganic species (i.e. metal ions) alter their composition when being electrosprayed. Parts of the weakly complexing ligands will be exchanged by solvent molecules, mainly originating from the sheath liquid. The destruction of ion-solvent clusters by heating, collision-induced decomposition or use of a sheath gas may lead to charge reduction of transition metal ions. Organometallic complexes with strongly complexing ligands remain intact, while those with weakly complexing ligands suffer from the same disadvantages as inorganic species. ESI-MS is best suited for the speciation of covalent organometallic compounds. The ionisation process does not alter their structure and they will mostly be detected as singly charged molecular ions. The application of CE-ESI-MS for selenium speciation to an existing method using an alkaline buffer system (Na 2 CO 3 -NaOH) gave unsatisfactory results. The non-volatile electrolyte affects the ESI process dramatically. The final CE method used an acidic background electrolyte (2% acetic acid) for the separation of three organometallic selenium species [selenomethionine (SeM), selenocystamine (SeCM) and selenocystine (SeC)]. The Se species were sufficiently separated from each other and appeared at 6.49 min (SeCM), 19.47 min (SeM) and 20.60 min (SeC). Detection limits were calculated as 1-6 mg/l for the organic Se species.