Mercury (Hg) is among the most conced contaminants in the world.It has three major chemical forms in the environment,including Hg0,Hg2+,and methylmercury (MeHg).Due to their differences in toxicity,mobility,and bioavailability,speciation analysis is critical for understanding Hg cycling and fate in the environment.SnCl2 reduction-atomic fluorescence spectrometry detection is the most commonly used method for analyzing inorganic Hg.However,it should be noted that MeHg may also be reduced by SnCl2,which would result in the overestimation of inorganic Hg.In this study,the reduction of MeHg by SnGl2 in both de-ionized (DI) water and four natural waters was investigated.The results showed that MeHg could be reduced by SnCl2 in DI water whereas this reaction was hard to occur in tested natural waters.By investigating the effects of water chemical characteristics (dissolved organic matter,pH and common anions and cations) on this reaction,SO42-was identified to be the dominant factor prohibiting SnCl2 induced MeHg reduction in natural waters.SO42-in natural waters was evidenced to be reduced to S2-by SnCl2 and the generated S2-can complex with MeHg to form MeHgS which is hard to be reduced by SnCl2.Findings of this study indicate that the effect of MeHg reduction by SnCl2 on inorganic Hg analysis is negligible in natural waters;however,at simulated experimental systems without SO42-,SO42-should be added as protecting agents to prevent MeHg reduction when analyzing inorganic Hg if it would not cause any other unwanted effects.