The tetravalent and hexavalent selenium content of different environmental water types can be determined after preconcentration on activated carbon by simple energy-dispersive X-ray fluorescence. The elemental selenium Is adsorbed on the activated carbon after specific reduction of the selenite by /-ascorbic acid. The total selenium content is determined after refluxing the water samples with thiourea in sulfuric acid medium and subsequent adsorption of the elemental form. The contribution of hexavalent selenium is obtained by subtraction. All of the experimental reduction reaction parameters have been optimized and the influence of humic material, salts, and oxidizing substances is investigated. The limit of detection is 50 ng L-1 for the tetravalent selenium and 60 ng L~1 for the total selenium content. The coefficient of variation amounts to approximately 10% for both species at the 0.5-1 pg Se L~1 level. The suspended material can easily be analyzed separately.