Dissolved organic matter (DOM) interacts very strongly with mercury, affecting its speciation, solubility, mobility, and toxicity in the aquatic environment. Strong binding of mercury by DOM is attributed to coordination of mercury at reduced sulfur sites within the organic matter, which are present at concentrations much higher than mercury concentrations found in most natural waters. The ability of organic matter to enhance the dissolution and inhibit the precipitation of mercuric sulfide, a highly insoluble solid, suggests that DOM competes with sulfide for mercury binding. This is confirmed by very high conditional stability constants for mercury-organic sulfur (RSHg+) complexes (10(25)-10(32)) recently reported in literature. DOM appears to play a key role in the photochemical reduction of ionic mercury to elemental mercury and subsequent reoxidation of elemental mercury to ionic mercury, thus affecting volatilization loss and bioavailability of mercury to organisms. DOM affects the production and bioaccumulation of methylmercury, the most bioaccumulative mercury species in fish.