The Illinois Basin of the Midwestern United States is an intracratonic basin containing substantial coal reserves in Upper Carboniferous strata. The rank of this coal is anomalously high, considering that the coal was not buried deeply. A reexamination of paleotemperatures at the time of coalification (using vitrinite reflectance; conodonts; and fluid inclusions) indicates distinct patterns. In the southern portion of the basin, paleogeotherms (profiles of paleotemperature vs. depth) are irregular. In the central part of the basin, there is an abrupt increase in the geothermal gradient at the level of the Pennsylvanian/Mississippian contact. In the northern portion of the basin, the gradient is parallel to present-day geotherm. These results are compatible with a model in which hot fluids, migrating from south to north through aquifers within the Carboniferous, introduced heat to the coal. These fluids may have been warmed by Permian intrusions at the south of the Illinois basin. Preliminary finite-element models of groundwater flow, produced using Basin2, demonstrate that such fluid migration could produce temperatures necessary to produce coal ranks observed. Preliminary cathodoluminescence study found baroque dolomite in cements, further evidence of hydrothermal-fluid migration. Fluid inclusions in the veins indicate fluid temperatures were close to 100°C.