The present paper is a part of the studies on the structure and properties of the Zonguldak bituminous coal samples, from the Kozlu K20 G borehole. The changes in the values of parameters characterizing the properties of the tested coals with an increase in the degree of coalification at increasingly greater depths in the borehole were reported. Changes in elemental and proximate analyses results, calorific value, FTIR aromatic bands, ESR values, and aromaticity of the samples were described.