•Co-pyrolysis of cornstalk with lignite and bituminous coal was studied.•Characteristic values and stages of all single and blended samples were identified.•Slight synergistic effects were evidenced by both TG/DTG and FTIR profiles.•0.3–7.4 % higher final residual yields than expected were confirmed in co-pyrolysis.•Thermal behavior of biomass was influenced by the presence of coal at 286–306 °C. Co-pyrolysis characteristics of cornstalk with two types of coal (lignite, bituminous coal) were investigated using thermogravimetry coupled with Fourier transform infrared spectrometry (TG-FTIR). Pyrolysis thermal behaviors of biomass and coal samples and their blends in different blending ratios were revealed by TG and DTG profiles, and an online monitoring of gas products evolved was realized by FTIR measurement. In order to explore potential synergistic effect, characteristic values of TG and DTG curves were identified for all blended and parent samples. In the meantime, evolution characteristics of CO2, CO, CH4, H2O and formic acid were identified by FTIR profiles against temperature. Slight synergistic effects were approved by both TG and FTIR analysis, which resulted in higher char yields and influences on volatile evolution during co-pyrolysis. 0.3∼7.4 % higher final residual yields than expected were confirmed in co-pyrolysis. TG results showed that thermal behavior of biomass was remarkably influenced with the presence of coal in blended samples at 286–306 °C. FTIR profiles also indicated that the evolution of formic acid was affected according to the releasing characteristics of CO and C–O groups at the same temperature region. SEM images and BET analysis of residual char provided further information about synergy. Disparity of thermal behaviors and void spaces between parent biomass and coal brought favorable conditions for adsorption and coking of both biomass and coal volatiles, which led to different gas-solid interactions and significantly changed the surface morphology and porous structure of co-pyrolyzed char particles.