The purpose of this study was to examine the levels of As and Hg and mobility of these contaminants in the sediments of the River Tisa and canal sediments (alluvial formation of the Danube River, Serbia), in order to determine the degree to which the ecosystem is harmed by these pollutants. The sequential extraction procedure (modified Tessier method) was used to extract the metals from the sediments. Arsenic is extracted in the second, third and fifth fraction, with dominant extraction in the second fraction. This distribution indicates that As is significantly present in the form of carbonates, whereas the part of As is present in the form of oxides. The most important extraction of Hg is in the fifth stage (extraction with 6M HCl), with over 80 % of extracted element in both types of sediment, indicating a strong association between Hg and Fe crystalline oxides as well as presence of Hg in the form of sulfides. Based on arsenic and mercury content in sediments and results of sequential extraction it can be concluded that on the studied localities there is no significant As and Hg contamination.