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Highlights • Complexation of Hg with the antibiotics resulting in the dissolution of Hg compounds from the sediment. • Hg ions in the sediment can also promote the degradation of antibiotics. • The degradation of TC/OTC radicals generates CH3+ groups which can react with Hg° to form MeHg. Abstract The role of antibiotics commonly used in fish culture activities in methylmercury (MeHg) formation in mariculture sediments (MS) and in reference sediments (RS) was studied using simulation microcosms. MS and RS were split into three equal batches. Two batches were spiked with Hg(NO 3) 2 aqueous solution at levels of 2 and 8 mg kg−1 (dry weight basis) and the remainder served as a control batch. Tetracycline (TC) and oxytetracycline (OTC) (2.5 g and 10 g of each) were added to each treatment. Sediment THg concentration decreased during the culture period possibly due to complexation of Hg with the antibiotics resulting in the dissolution of Hg compounds from the sediment. More importantly, the MeHg concentration increased after 32 days together with a decrease in the concentrations of the antibiotics in the sediment. The complexation of TC or OTC with Hg resulted in the transport of electrons from TC or OTC to Hg2+ due to the high electronegativity of Hg2+. Subsequently, Hg2+ was reduced to Hg° which reacted with CH 3 + derived from TC or OTC. The use of antibiotics may therefore promote the formation of MeHg in sediments. [ABSTRACT FROM AUTHOR]