In this work, 13 selenium species in flue gas desulfurization (FGD) waters from coal-fired power plants were separated and quantified using anion-exchange chromatography coupled to inductively coupled plasma mass spectrometry. For the first time, we identified both selenosulfate (SeSO(3)(2-)) and selenocyanate (SeCN(-)) in such waters, using retention time matching and confirmation by electrospray mass spectrometry. Besides selenite and selenate, selenosulfate was the most frequently occurring selenium species. It occurred in most samples and constituted a major fraction (up to 63%) of the total selenium concentration in waters obtained from plants employing inhibited oxidation scrubbers. Selenocyanate occurred in about half of the tested samples, but was only a minor species (up to 6% of the total selenium concentration). Nine additional Se-containing compounds were found in FGD waters, but they remain unidentified at this point.