Enrichment of organic matter with sulfur during early diagenesis has been amply documented in marine sediments. The importance of such reactions in lacustrine sediments is not as well appreciated. In this study the organic sulfur contents of sediments from seven lakes were compared, and the speciation of the sulfur in the humic acid fraction of the sedimentary organic matter was examined with X-ray photoelectron spectroscopy (XPS). Diagenetic enrichment of organic matter with sulfur occurred in five of seven lakes as evidenced by C:S ratios and stable isotope ratios. The availability of sulfide, organic matter, and reactive iron were not important determinants of the extent of organic S enrichment. The only environmental parameter that appeared to be related to S enrichment was lake trophic state. Together with data from the literature, our results suggest that sediments in most eutrophic lakes are enriched in organic S, while organic matter is enriched in only few oligotrophic lakes. Organic sulfides or thiols are the dominant forms of reduced organic S in the humic acids, and sulfur enrichment occurs primarily by formation of organic sulfides or thiols. Addition of S to polyunsaturated molecules could account for 5-10% of the S enrichment. Di- and polysulfides did not comprise a significant fraction of organic S in any of the sediment humic acids. Thiophenes could be identified tentatively only in the oldest (60 yr) sediment sample analyzed. Sulfoxides were observed in several samples. Lake trophic state and exposure to oxygen appear to be major factors influencing the extent and pathways of S addition to organic matter.