Selenium(Se)speciation is essential for understanding its potential bioavailability and impact on the environment.The distribution and possible transport pathways of Se in soils have been comprehensively investigated,but little is known about the chemical speciation of Se in Se-rich soils in Enshi,where a sudden incidence of human selenium(Se)poisoning occurred.The combined techniques of sequential extraction procedure(SEP)and X-ray absorption fine structure(XAFS)was utilized to study Se speciation in natural samples from selenosis area.The SEP results showed that Se fractions in Se-rich soils from Enshi can be dominated by the organic matter associated Se(OM-Se)and/or sulfide/selenide Se,which accounted to 48.2-66.6%and nearly 20%of total Se in soil,respectively.The XAFS result suggested that Se(Ⅳ)is the predominant form(46.4-72.6%of total Se),and XAFS provide direct evidence for the fact Se can be oxidated from reduced form(Se(-Ⅱ,0))during process of rock weathering in the high-Se supergene environment.Se speciation determined by SEP is in generally agreement with the result obtained by XAFS.However,it should take some caution with SEP data for sample with high concentration of selenide/sulfide Se.The combination of SEP and XAFS can identify and quantify Se speciation with high reliability and precision.