The redox state of Lake Sihetun, represented by the Lower Cretaceous Yixian Formation (western Liaoning, China), is evaluated to understand the formation of this Konservat-Lagerstätte. Lake evolution is subdivided into four phases, of which Phases 2 and 3 exhibit excellent fossil preservation. Exceptional preservation and mass mortality events within Phase 2 were previously attributed to synsedimentary volcanism and oxygen deficiency. However, the volcanic trigger for mass mortality events remains enigmatic and distinction between anoxia and dysoxia has not been put forward so far. To resolve the redox state of the lake during Phase 2, 5394 diameters of pyrite framboid pseudomorphs in 29 thin sections from three localities have been measured. Framboid size distributions reveal that Lake Sihetun was governed by dysoxic bottom waters with spells of anoxia, recording pronounced environmental stress. Hence, holomictic, eutrophic conditions were episodically replaced by meromictic interludes. Spatial variations in redox state were common and oxic conditions were detected in two of the three studied localities. Concentrated iron sulphide layers resulted from biofilms at the lake floor or the development of microenvironments around organic remains, which preserved highly reactive organic compounds. In contrast, sediments of Phase 3 record oxic conditions and an entirely holomictic lake.