We have assessed the effect of various factors on the mobility of total mercury (Hg) as well as on the methylation of Hg in two floodplain soils with different Hg loads using an automated biogeochemical microcosm system allowing controlled variation of redox potential (EH) in soil suspensions. The experiment was conducted under stepwise variation from reducing (approximately -240 mV at pH 5) to oxidizing (approximately 600 mV at pH 5) conditions. Generally it is known that sulphate (SO42-) reduction is mainly catalyzed by sulfate reducing bacteria (SRB) and iron (Fe) reducing bacteria, which promote the Hg methylation. However, the specific linking mechanisms between Hg methylation and microorganisms are still a black box. Analysis of phospholipid fatty acids (PLFA) indicate the occurrence of sulphate reducing bacteria such as Desulfovibrio or Desulfobulbus species, which are considered to promote Hg methylation. A direct impact of EH and pH could not be detected, instead, EH and pH indirectly affected Hg dynamics and methylation rate. Dissolved organic carbon (DOC) is an important factor controlling the dynamics and methylation of Hg. Both, Hg and methyl mercury (MeHg) in the solution increased with raising DOC. The lnMeHg/Hg ratio as the net production of MeHg normalized to the Hg concentration increases with increasing lnDOC/Hg ratio under changing EH.