Geochemical lability of trace metals in sediments is crucial for accurate environmental risk assessment. In periodically flooded sediments, the complicated reactions among sediment components induced by changing REDOX conditions and the limitation in analytical methods make it difficult to quantify the metal lability or to identify the dominant factor that controls the lability. We proposed a modified isotopic dilution method, which measures metal lability (known as the ‘E-value’) with minor disturbance of REDOX potential (Eh) during the experimental process. Contaminated sediments with different organic matter contents (OM%) were incubated in laboratory under reducing conditions (by adding N₂), and the fluctuation of Cd, Cu, Pb and Zn lability (E%=E-value/total concentration) were illustrated in response to the reducing Eh. The E_Cd% fluctuated the most among all four metals: from +90mV to -45mV, the E_Cd% decreased by 22%; then elevated by 23% before the Eh reached -135mV; and finally decreased sharply by 21% till Eh=-165 mV. For Cu, Zn and Pb, the similar pattern of E% fluctuation implies that the controlling mechanism are the same for these 3 metals at different stages of decreasing Eh. The combined effect of metal release from mineral oxides, sorption on solid organic matter and complexation with dissolved organic matter leads to an intermittent increase in E% for these 3 metals in the Eh range from 90mV to -135mV. Sharp decrease in E% was observed for all metals in anoxic condition as a consequence of precipitation of stable metal sulphides. Cd lability in sediments with lower OM% is more ‘sensitive’ to the reducing condition than that in higher OM% sediment. With initial E_Cd%=46% in oxic condition, the value reached up to >94% in the range of 0mV to -50mV. In contrast, the effect from OM% was much less significant for Cu, Zn and Pb when the Eh decreased from 120 mV to -70 mV. According to this study, during the flooding period, the environment may experience highest risk from toxic trace metals in sediments when it reaches suboxic condition. The risk diminishes when it becomes anoxic as the formation of sulphide minerals can largely reduce the lability. Among all 4 metals, Cd should be given more attention especially in sediments with low OM content. The modified isotopic dilution method is suggested for future investigation on the influence from other sediment components, e.g. Fe and Mn oxides, carbonates, etc. on trace metal lability under altered REDOX conditions.