To investigate the effect of mineral matrix on thermal degradation of Goynuk oil shale and Sirnak asphaltite, their samples (initial ones and those subsequently treated with solutions of the acids: HCl, HNO3 and HF) were studied in a thermogravimetric analyzer. Initial and HCl-washed samples showed similar degradation behavior, while HNO3 washing affected it in both cases. Changes in organic structure, which is different for shale and asphaltite the former being aliphatic, and the latter having aromatic character after HNO3 treatment affected the degradation kinetics of the samples differently, whereas the mineral matter content had no effect on it.