Oil shale ash is a material that could be used as a low cost catalyst for biomass gasification processes. This article analyses the catalytic effect of oil shale ash on CO2 gasification of reed and leached wheat straw chars. A thermogravimetric analyser operated at atmospheric pressure and two different partial pressures of CO2 such as 0.09 atm and 1 atm were applied. The results indicate that oil shale ash has positive effect on char reactivity at the CO2 partial pressure of 0.09 atm: the addition of oil shale ash by 30% to reed char lowered the reaction time by 1.3 times and to leached wheat straw char by 1.4 times. At the partial pressure of CO2 1 atm, the addition of oil shale ash resulted in the negative effect on the char conversion due to the binding of CO2. The average calculated CO2 binding capacity of oil shale ash was 13.7 mg CO2 per 100 mg of oil shale ash. (C) 2018 Elsevier Ltd. All rights reserved.