Purpose This study explored the effects of anoxia on adsorbed Si dynamics in surface sediments. While the effects of anoxia on the adsorption and release of phosphorus (P) have been studied frequently, its effects on adsorbed silicon (Si) are commonly ignored, despite the similarities between the elements. The study addresses the theory that as both Si and P produce anionic ligands, processes that affect adsorbed P and its release could also affect Si and its release. Materials and methods Two main procedures were conducted: A sequential extraction procedure composed of (1) water, (2) 0.46 M sodium chloride, (3) 0.11 M sodium dithionite, (4) 0.1 M sodium hydroxide and (5) 0.5 M hydrochloric acid extractions, and a long-term release dynamics experiment (RDE) lasting 28 days. Both were conducted using seven sediment samples of different composition. Results and discussion Silicon behaved differently than P under anoxic conditions. Although anoxia generally causes increased release of Fe-bound P due to Fe 3+ reduction, the release of Si decreased under anoxic conditions in the loosely sorbed and redox-sensitive fractions of the sediments. The long-term experiment (RDE) showed that Si release was time-dependent and slower under anoxic than oxic conditions. Resorption to Al oxides and the formation of colloidal mixed Fe-Si oxides are possible reasons for these results. There were, however, also some indications that anoxic conditions enhanced biogenic Si (BSi) and mineral Si dissolution. Conclusions Anoxia clearly affects Si sorption processes, but it may have both negative (e.g. increased resorption preventing P adsorption) and positive (e.g. enhanced BSi dissolution, increasing dissolved Si concentration in water) effects on the release of Si. More investigations dealing with the subject are needed.