Tests of coal samples collected from a borehole drilled in the surrounding of an underground coal gasification georeactor were conducted. This experiment was conducted for the first time in an active coal mine in Poland. The aim of the tests was to determine the extent of heat penetration around the UCG cavity during a nearly two month gasification process. The method of measuring the mean reflectance of the vitrinite index (R-o), which is more generally used to measure the degree of the coalification of organic substances in coal (macerals) under the influence of temperature, was applied. The method is still used to evaluate the usefulness of coal for producing coke. The mean reflectance of the vitrinite index determined for the collected samples and reflectograms, allowed the inference that when the georeactor was operating, the temperature in the UCG cavity reached approximately 1300 degrees C. Based on Ro, the point in a coal seam surrounding the georeactor corresponding to the temperature of approximately 700 degrees C was also identified, where there were signs of the initial stage of coal transformation. The point was located 1.94 m from the boundary of the UCG cavity. Then, at the distance of 1.73m from the boundary of the UCG cavity the point with a temperature of approximately 1000 degrees C was identified. The tests confirmed that it is possible to use the method of determining the mean reflectance of vitrinite index to measure the distance of influence of georeactor temperature on the rock mass. This is particularly important for underground coal gasification conducted in active mines. In the future the results will be used to predict the time of heating needed for coal deposits to become gasified, based on laboratory tests and coal type.