Molybdenum(Mo)is a relatively special transition element due to its major different concentrations in continental crust(1.2 ppm)and the ocean(10.3 ppm).Molybdenum is also a crucial tool for scientists to study palaeo-environment.The weathering of black shales experienced various processes such as redox,degradation of organic matters and development of iron and manganese oxides,which affect Mos association and release.Sequential extraction procedure provides an effective method to acquire the information on Mo fractions.We used improved 5-step sequential extraction procedures(including F1 exchangeable,F2 organic matter,F3 Fe-Mn oxides and carbonates,F4 sulfide and F5 residue)to extract different Mo associations of black shales collected from Enshi,Hubei,China.Three differently weathering extent(no visible weathered,some weathered and strongly weathered)of black shales were tested.Molybdenum concentrations of each fractions and total Mo were determined by ICP-MS.There are good correlations between total Mo and the sum of Mo fractions in each sample.Our data indicated that percentage of exchangeable Mo was higher in some weathered samples; percentage of Mo bound to organic matters is the highest in fresh black shale,and the percentage of Mo bound to sulfides is almost not changed.The Mo fractions in the weathering system of black shales transfer from one fraction to another during weathering processes.As weathering intensified,Mo bound to organic matter,sulfide and carbonate or oxides transferred to exchangeable.Weathering lead Mo bound to organic matters,sulfide and carbonate or oxides released while exchangeable Mo was abounded.Molybdenum bound to sulfides was lost at the beginning of weathering but it might be enriched in reducing zone as Mo released from organic matters or exchangeable.The geochemical behavior of Mo in weathering system of black shales is active.The whole process is affected by pH,Eh and some other potential factors such as microbes,Fe(Mn)oxides developed during weathering processes.