Abstract The most recent sedimentary sequence (ca. < 200 yr) of Lake Greifen in northeastern Switzerland shows evidence for an oxic/anoxic depositional-environment transition dated at 1930. Organic matter is composed predominantly of autochthonous algal and amorphous material with minor contribution of allochthonous terrestrial material. A change in the chemical composition and concentration of organic matter across the laminated-marl/seasonally varved organic-rich sediment transition is characterized by decreasing O/C and increasing H/C ratios, Hydrogen Indices, TOC, organic carbon accumulation rates, and petroleum potential (kg Hydrocarbons/ton bulk rock). Variations in the quality and quantity of organic matter deposited are synchronous with measured changes in water-column-nutrient concentrations, surface-water productivity, and, most importantly, the thickness and extent of the oxygen depleted water mass. The lacustrine algal source material deposited under predominantly anoxic waters (seasonally varved sediments) is hydrogen rich and resembles chemically type I kerogens and organic facies “A” to “AB” according to Jones’ classification (Jones, 1987), while organic matter deposited under primarily oxygenated waters (laminated-marl sediments) shows H/C and O/C ratios and HI values resembling those of type II or III kerogens and organic fades “B” to “BC”. The correlation between changing composition and concentration of deposited organic matter and the historical water-column geochemical record documents how quality and quantity of organic matter can be significantly altered, depending upon the relative thickness of the oxygen-depleted water mass at the time of deposition. Variability in the efficiency of organic-matter decomposition during settling and at the sediment-water interface, in association with a varying redox potential of the oxygen-depleted water mass and in the sediments, is perhaps the ultimate control on the composition and concentration of sedimentary organic matter and the determination of organic facies (Jones, 1987). The deposition of high-quality lacustrine organic facies “A” was constrained stratigraphically to a time interval when the water column was severely depleted in oxygen and sulfate and the sediments were permanently anoxic. Moreover, the occurrence of methane in the water column at this time suggests that fermentation processes in the permanently anoxic sediments and possibly in the stratified water- column were probably dominating organic-matter decomposition. Carbon isotopic analyses of individual branched and cyclic hydrocarbons further suggest that methanotrophic and chemoautotrophic bacteria were present in the uppermost sediment layers and the severely anoxic waters. We postulate that, during times of widespread depletion of oxygen and sulfate in the water column and sediments, the predominance of bacterial fermentation processes controlling organic-matter degradation and the addition of a lipid-rich bacterial biomass to the sediments resulted in the occurrence of a highly preserved organic facies “A” and/or sediments of an excellent petroleum potential. The predominance of an anoxic and sulfate-depleted water column and the existence of permanently anoxic sediments are perhaps the environmental conditions necessary to explain the common occurrence of highly preserved organic facies “A” in modern and ancient lacustrine settings and their rare occurrence in marine environments.