Effects of biomass on the pyrolytic decomposition of different rank coals were investigated by non-isothermal Thermogravimetric Analysis (TGA) method from ambient to 900 °C with a heating rate of 40 °C/min under nitrogen. Hazelnut shell (HS) which is a woody biomass species was added as much as 10 wt% to coals such as peat, lignite, bituminous coal, and anthracite to obtain coal/biomass blends for co-pyrolysis runs. Effects of HS present in the blends were evaluated regarding the apparent decomposition rates and the char yields. It was found that the addition of thermally reactive HS led to some increases in the volatilization rates of coals especially at temperatures below 500 °C. Besides, the char yields revealed unexpected variations in case of low rank coals. Although, HS addition did not play significant role on the char yields of bituminous coal and anthracite, considerable deviations from the theoretical char yields were detected in the case of peat and lignites. The presence of HS led to increasing char weight for peat, while the char weights for lignites decreased seriously. These variations were interpreted, and it can be concluded that these variations cannot be explained by simple additive behavior, and the existence of synergistic interactions should be taken into account. © 2009 Elsevier Ltd. All rights reserved.