Abstract Investigations into the effects of mineral matter content on Hardgrove Grindability Index (HGI) were carried out on some low-rank Turkish coals. Quantitative X-ray diffraction (XRD) analyses were carried out using an interactive data processing system (SIROQUANT™) based on Rietveld interpretation methods. Selective leaching processes were used to determine the water and acid-soluble contents of coal samples. Among the coal seams tested, the HGI values of Elbistan coal samples presented a large range from 39 to 83, whereas Tufanbeyli coal samples ranged from 48 to 69. Treatment of the coal with water, ammonium acetate, and hydrochloric acid showed that a considerable part of the ash-forming inorganic matter occurs in water-soluble, acid-soluble, or ion-exchangeable form. Grindability tests on samples of varied water and acid-soluble content showed a significant effect of water and acid-soluble contents on HGI.