The recent research is focused on the experimental study and mathematical modelling of the development of combustion dynamics at thermo-chemical conversion of biomass mixtures (straw with wood pellets) with the aim to better understand the effect of straw co-combustion with wood pellets on the formation of the main gasification/ combustion characteristics. The results of experimental study have shown that thermal interaction between the components at co-combustion of straw with wood pellets at average mass load of straw in the mixture up to 20-30 % promotes faster thermal decomposition of the mixtures and accelerates the flaming combustion of volatiles. The mathematical model considers development of two second order exothermic irreversible chemical reactions at chemical conversion of combustible volatiles (H2, CO) to assess their influence on the development of the combustion dynamics downstream the reacting swirling flame flow. The results of mathematical modelling have shown that, in accordance with the data of the experimental study, the maximal values of the flame temperature, axial flow velocity and the mass fractions of the main products (CO2, H2O) at the thermo-chemical conversion of the biomass pellets and their mixtures were obtained at 20-30 % of the mass load of straw in the mixture, which is recommended as optimal composition of the mixture.