The gasification/combustion characteristics of biomass pellet mixtures were experimentally studied and mathematically modelled by co-firing wheat straw with peat with the aim to assess the effect of straw addition to peat on the mixture thermal decomposition, on the formation and combustion of volatiles, and to optimize the mixture composition and to provide more intensive use of straw for energy production. The results of the experimental study show that the co-firing of straw with peat pellets enhances the thermal decomposition of peat pellets at the primary stage of the flame formation (t < 500 s). This leads to more intensive release of the combustible volatiles entering the combustor, to faster ignition of volatiles and faster formation of the flame reaction zone. Analysis of the effect of wheat straw co-firing with peat has shown that the thermal interaction between the components completes the combustion of the mixture, increasing so the heat output from the device, the produced heat energy, the volume fraction of CO2, whereas the air excess in the products decreases, with the maximum co-firing effect on the main combustion characteristics at 10-20 % of the straw mass load. A mathematical model for volatiles (CO, H2) combustion downstream the combustor has been developed using the environment of MATLAB package and two exothermic reactions for H2 and CO combustion with account of variations in CO:H2 supply into the combustor by varying the mass load of straw in the mixture.