A comprehensive study was conducted in July 2006, January 2007 and March 2007 to determine the impacts of some major physicochemical parameters on the level of mercury (Hg) in Puding Reservoir, Guizhou, China. The concentrations of Hg species in the summer campaign were significantly higher (p<0.01, generally 2 to 3 times higher) than those in the winter and spring campaigns, and no statistical differences were found between the same parameters for the latter two campaigns (p>0.05). Ancillary parameters including suspended particulate matter (SPM), dissolved organic carbon (DOC), temperature (T), dissolved oxygen (DO), pH, nitrate (NO3-) and chloride (Cl-) were also measured. During the sampling campaign in July 2006, average values for SPM, DOC, T, and NO3- were all higher compared to the other two campaigns, which suggested a similar seasonal trend between these parameters and Hg species. Seasonal variability may be related to increased runoff. High runoff volume due to abundant precipitation in the summer carried Hg-laden particulates into the reservoir, whereas there was less precipitation in the winter and spring when Hg levels were lower. Increased agricultural activity in the summer season also increased Hg levels in Puding Reservoir.