The results of a reconnaissance study of 58 samples of Pennsylvanian carbonate beds, collected over a wide range of areas within the state and the whole stratigraphic section, reveal that significant quantities of ferroan-dolomite are present. Methods of analysis were: (1) X-ray diffraction patterns for estimation of relative abundance of calcite and dolomite structures, (2) differential thermal analysis for identification of ferroan-dolomite, and (3) thin section petrology for characterization of textures and relationships between carbonate minerals in the rocks. Some representatives of all limestone beds below the Trivoli limestone (mid-McLeansboro group) contain ferroan-dolomite; higher limestones are almost completely calcitic. The limestones in the lower part of the McLeansboro group, particularly the Bankston Fork, consistently show some high ferroan-dolomite percentages. There is no apparent correlation of dolomite content with basin or shelf occurrence, or surficial weathering. Study of thin sections shows that dolomite rhombs are secondary and are most abundant in association with certain rock textures. Suggestions are made for further study on problems of mineralogy and genesis of limestone and dolomite beds.