The limestone injection process is an effective, inexpensive way to control SO2 in flue gases. Thus detailed petrographic, mineralogical, and chemical analyses of 26 carbonate rocks were made and compared with the capacity (3 1/2 hr. reaction period) and differential reactivity (120 sec. reaction period) of calcined specimens for sorption of sulfur dioxide (SO2). The study also included petrographic and chemical examination of limestone-modified fly ashes and boiler deposits. Image analysis and scanning electron microscope methods were employed to examine limestones, dolomites, and a variety of other types of carbonates and their calcines. A wide range of petrographic and SO2 sorptive properties were revealed. The relatively high SO2 reactivity observed for chalks, calcareous marls, and oolitic aragonite sand samples is believed due mainly to the high pore volume and fine grain size of these carbonate rocks.