Redox processes can significantly affect the acid-producing potential and metal bioavailability in sediments. Many oxidation reactions producing acid can occur in natural aquatic sediment systems and human-affected environments. S, Fe, and N are the most important elements involved in redox processes of sediment/water systems. The actual extent of acidification in a sediment/water system depends not only on APP, but also on the acid neutralizing capacity (ANC) of the system. Periodic redox processes leading to "ferrolysis", "split" of sulfate, and the volatilization of H2S can cause changes in APP, and then affect metal transformation and bioavailability. The effects of redox and pH variations on the mobility of metals has been successfully assessed by pore water chemistry, which reflects the interactions of metals in aqueous and solid systems under changing redox environments with depth of sediments. Redox reactions, especially reoxidation of anoxic sediments, can lead to metal release from contaminated sediments. The bioavailability of metals in sediments depends on metal species, especially metal sulfide species. A lack of understanding of the kinetic processes controlling acid production and about the accurate determination of ANC quantitatively limits our ability to predict apparent APP. Heavy sulfide (AVS) may control the toxicity of heavy metals in anoxic sediments, under changing redox conditions AVS is oxidized and dissolved metal sulfates are released. Furthermore, the acidity which follows sulfide oxidation may accelerate metal release. Thus, assessment of the relationship between metal sulfides and their bioavailability in periodic redox environments needs much more research.