Abstract: A comparative study was conducted to investigate selenium fractionation in Serich carbonaceous rocks from the lower Cambrian Niutitang and Permian Maokou formations, which are located in Zunyi of Guizhou Province and Enshi of Hubei Province, respectively. Seven operationally defined Se fractions in Serich carbonaceous rocks were determined by the modified sequentialextraction procedure suitable for carbonaceous shale. These fractions include watersoluble, ligandexchangeable, basesoluble (organicbounded), elemental, acetic acidsoluble, sulfide/selenide, and residue. The total Se in rock and Se concentrations in individual fractions were measured by HGAFS. The results showed that the sulfide/selenide and organicbounded Se were the predominant forms of Se present in the carbonaceous chert and carbonaceous shale from the lower Permian Maokou formation while residue Se was generally lower. The sulfide/selenide and organicbounded Se were still the major forms in carbonaceous chert of the lower Cambrian Niutitang formation while the organicbounded, residue and sulfide/selenide fractions were prevalent in carbonaceous shale and NiMo ore. Organicbounded Se and elemental Se were the dominant forms in bentonite, and in lesser amount the ligandexchangeable Se. These facts indicated that there exists certain difference in the biogeochemical process of Se accumulation between two rock types from Lower Permian Maokou and Cambrian Niutitang formations. For the former formation, the Se microbe reduction was prevalent in the carbonaceous chert, while in the carbonaceous shale the algal assimilation or adsorption occurred simultaneously with microbe reduction. For the later formation, the algal assimilation or adsorption was be the main biogeochemical process for Se enrichment, while the Se reduction by microbes played a secondary role.