Abstract. An automated biogeochemical microcosm system allowing controlled variation of redox potential (E_H) in soil suspensions was used to assess the effect of various factors on the mobility of mercury (Hg) as well as on the methylation of Hg in two contaminated floodplain soils with different Hg concentrations (approximately 5 mg Hg kg⁻¹ and >30 mg Hg kg^–1). The experiment was conducted under stepwise variation from reducing (approximately −350 mV at pH 5) to oxidizing conditions (approximately 600 mV at pH 5). Results of phospholipid fatty acids (PLFA) analysis indicate the occurrence of sulfate reducing bacteria (SRB) such as Desulfobacter species (10Me16:0, cy17:0, 10Me18:0, cy19:0) or Desulfovibrio species (18:2ω6,9), which are considered to promote Hg methylation. The products of the methylation process are lipophilic, highly toxic methyl mercury species such as the monomethyl mercury ion [MeHg⁺], which is named as MeHg here. The ln(MeHg/Hg_t) ratio is assumed to reflect the net production of monomethyl mercury normalized to total dissolved Hg (Hg_t) concentration. This ratio increases with rising dissolved organic carbon (DOC) to Hg_t ratio (ln(DOC/Hg_t) ratio) (R² = 0.39, p<0.0001, n= 63) whereas the relation between ln(MeHg/Hg_t ratio and lnDOC is weaker (R² = 0.09; p<0.05; n = 63). In conclusion, the DOC/Hg_t ratio might be a more important factor for the Hg net methylation than DOC alone in the current study. Redox variations seem to affect the biogeochemical behavior of dissolved inorganic Hg species and MeHg indirectly through related changes in DOC, sulfur cycle, and microbial community structure whereas E_H and pH values, as well as concentration of dissolved Fe³⁺/Fe²⁺ and Cl^– seem to play subordinate roles in Hg mobilization and methylation under our experimental conditions.