It is well known that chlor-alkali industry has traditionally used the mercury (Hg)-electrolysis process to produce mainly chlorinated solvents. Unfortunately, a fraction of the Hg used during production is released through drainage into the aquatic environment where it can be incorporated to biota. In Europe, Spain and Germany are leading in the number of plants which still use this technology. Moreover, it should be highlighted that three out of the eight chlor-alkali plants in Spain which are still operating with this process are located in the Ebro River basin in the proximities of Sabinanigo and Monzon cities – along the tributaries Gallego and Cinca Rivers, respectively – and Flix along the Ebro River. Therefore, the midlow Ebro River watershed might be considered as a hot spot of aquatic mercury pollution in Spain. This chapter focuses on all the information published up to date about total mercury (THg) and organomercury, with special emphasis on methylmercury (MeHg), in different aquatic organisms sampled along the Ebro River. First, a brief explanation of the current knowledge regarding the sources and cycling of Hg and its transformation into MeHg is presented. Later, in this chapter, THg and limited data on organomercury levels in aquatic organisms of the Ebro River basin are detailed. The aquatic organisms most commonly studied in the Ebro River basin are zebra mussel, red swamp crayfish, and different fish species, namely European catfish, northern pike, common carp, rudd, roach, barbell, and bleak. According to the different sentinel species analyzed, THg levels in specimens collected downstream from the impacted areas are 10–20 times greater than upstream levels. It clearly points out the relevance of chlor-alkali plants in terms of mercury river pollution. L. Carrasco, J.M. Bayona, and S. Diez ð*Þ Department of Environmental Chemistry, Institute of Environmental Assessment and Water Research, IDAEA-CSIC, Jordi Girona, 18-26, 08034 Barcelona, Spain e-mail: sdsqam@cid.csic.es D. Barcelo and M. Petrovic (eds.), The Ebro River Basin, Hdb Env Chem (2011) 13: 239–258, DOI 10.1007/698_2010_71, # Springer-Verlag Berlin Heidelberg 2010, Published online: 28 July 2010 239