We report a first investigation of the following dissolved forms of silicon in natural waters of Karelia: monomeric orthosilicic acid, polysilicic acids and silicon within dissolved organic compounds. A linear relationship was found between the concentration of the silicon-bearing organic compounds and indirect characteristics of organic matter content, with the correlation coefficient at 0.85. The seasonal variation of total dissolved silicon concentration and the ratio of its forms in different types of water bodies in Karelia were determined. The content of dissolved silicon was the lowest in spring and summer due to diatom plankton production. A technique has been suggested for tallying silicon within organic compounds in natural waters using persulfate oxidation in alkaline solution.