Methylmercury concentrations in fish from the historically polluted Minamata Bay remain higher than in fish from other coastal seas around Japan. To obtain a better understanding of this phenomenon, the chemical characteristics of pore water from Minamata Bay sediments were investigated. Samples were taken from two stations over a 1-year period. Total average values in the pore water at the two stations for dissolved total mercury and methylmercury concentrations were 6.64±4.93 and 2.69±2.07ng/l, respectively. The pore water was centrifuged at 1000rpm to 3000rpm. The highest ratio of dissolved methylmercury to dissolved total mercury exceeded 60% for pore water centrifuged at 3000rpm. Furthermore, because total average values of Log Kd of total mercury and methylmercury in sediment (St1 and St2) were 5.42 and 2.32 (Lkg−1), methylmercury in Minamata Bay sediment is more eluted than other mercury species. •Average-diss-total-Hg concentration in pore water was 6.64±4.93ng/l.•Average diss-methyl-Hg concentration in pore water was 2.69±2.07ng/l.•The highest diss-methyl-Hg content to diss-total-Hg ratio exceeded 60%.•Average Log Kd for total-Hg for the sediments was 5.42.•Average Log Kd for methyl-Hg for the sediments was 2.32.