The thermal reactivity and kinetics of five coal chars, a biomass char, and two coal/biomass char blends in an oxy-fuel combustion atmosphere (30%O2–70%CO2) were studied using the non-isothermal thermogravimetric method at three heating rates. Fuel chars were obtained by devolatilization in an entrained flow reactor at 1273 K under N2 and CO2 atmospheres. Three nth-order representative gas–solid models – the volumetric model (VM), the grain model (GM) and the random pore model (RPM) – were employed to describe the reactive behaviour of the chars. The RPM model was found to be the best for describing the reactivity of the high rank coal chars, while VM was the model that best described the reactivity of the bituminous coal chars, the biomass char and the coal-biomass blend char. The kinetic parameters of the chars obtained in N2 and CO2 in an oxy-fuel combustion atmosphere with 30% of oxygen were compared, but no relevant differences were observed. The behaviour of the blend of the bituminous coal (90%wt.) and the biomass (10%wt.) chars resembled that of the individual coal concealing the effect of the biomass. Likewise, no interaction was detected between the high rank coal and the biomass chars during oxy-fuel combustion of the blend. This work was carried out with financial support from the Spanish MICINN (Project PS-120000-2005-2) co-financed by the European Regional Development Fund (ERDF). M.V.G. and L.A. acknowledge funding from the CSIC JAE-Pre and CSIC JAE-Doc programs, respectively, co-financed by the European Social Fund. J.R. acknowledges funding from the Government of the Principado de Asturias (Severo Ochoa program). Peer reviewed