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Radon ( 222Rn) is a natural gas produced by the radioactive decay of uranium-bearing rocks and soils in the Earth’s crust. Geology is one of the main factors controlling source and distribution of radon. Most rocks and soils possess small amount of uranium, but there are particular types of bedrocks and unconsolidated deposits characterized by higher than average uranium contents. These lithological types include some granites, light-colored volcanic rocks, phosphatic rocks, ironstones, limestones and shales rich in organic matter as well as metamorphites derived from these rocks. They often contain minerals or phosphate and organic complexes in which uranium and radium occur. Highly fractured rocks or these containing solution cavities may also enhance radon transport and accumulation. The territory of Bulgaria includes a great variety of geological units with various rock composition, structure and evolution. However, complex studies concerning correlation between the measured radon concentrations and bedrock geology in Bulgaria are only scarce and very general. The present research aims to defining the rock formations in Bulgaria, in their outcrop presence, with possible high radon potential based on geological and published more general “radon – rock correlations” data. Based on that, an attempt for a GIS based map for the spatial distribution of the particular rock types in accordance with the expected radon potential is made. A part of these localities will be used for further radon measurements in order to identify the “radon – outcrop rock” correlations. The study will serve as a principal base for future evaluation of the radon potential and lithological rock type correspondence at the Bulgarian territory. [ABSTRACT FROM AUTHOR]