Hilt's law (dependence of the coalification degree on depth) is a substantial and long-acknowledged rule. Its validity in the Czech part of the Upper Silesian Coal Basin (Carboniferous, Mississippian to Pennsylvanian - Lower Namurian to Westphalian A) is the subject of this study. It is based on over 29,000 analyses of the volatile matter moisture- and ash-free (V-daf) from surface and underground boreholes drilled in 1946-1989. Vitrinite reflectance (R-max) cannot be used as a measure of coalification in the Upper Silesian Coal Basin: the number of reflectance measurements is small and their distribution over the basin area is very uneven. Statistical data on V-daf for the individual stratal units were processed. Modelling of the trends in the volatile matter moisture- and ash-free (V-daf) depending on "stratigraphic depth" proved the general applicability of Hilt's law. The distribution of this parameter in selected boreholes and segments of the basin was also studied, and coalification gradients were calculated. Coalification in specific examples (boreholes) is, however, highly variable and shows numerous deviations from Hilt's law even within individual boreholes, which is documented on a real example. The causes of some of the deviations are relatively well known (e.g., effect of elevated pressure in tectonically deformed areas, effect of effusive rocks or paleoweathering zones) while others can be presumed (heat flows due to Variscan and Carpathian orogenies affected by the lithology of the Carboniferous massif). The contribution of these effects in specific examples can be determined with great difficulty only. (C) 2007 Elsevier B.V. All rights reserved.