Diagenetic minerals and alteration patterns in the Ordovician St. Peter Sandstone, Illinois Basin, record varied hydrologic and chemical conditions during the basin?s long and complex geologic history. Major diagenetic events modifying the St. Peter Sandstone include (1) mechanical compaction, (2) early K-feldspar overgrowth and dolospar precipitation, (3) burial quartz, dolospar, anhydrite, and calcite cementation, and (4) carbonate-cement and K-feldspar grain dissolution. Radiometric age dates of authigenic K-feldspar and illite in combination with the reconstructed burial history of the St. Peter reveal that early-diagenetic K-feldspar and dolospar precipitated at shallow to moderate depths in the Devonian, whereas late-diagenetic quartz, dolospar, anhydrite, and calcite formed during deep burial in the Late Pennsylvanian to Early Permian. Stable-isotope geochemistry and fluid-inclusion paleothermometry suggest that burial cements precipitated from saline fluids over a wide temperature range. In the southern part of the basin, burial cements preserve a record of diagenetic effects that were in part controlled by fractures and hydrothermal-fluid circulation. Baroque dolospar cementation is the most significant of these effects.