The combustion behavior of different kinds of biomass (corn stalks, straw, rice husks, sawdust) and oil shale semi-coke and their blends was investigated. Non-isothermal thermogravimetric experiments were performed with different atmospheres (N(2):O(2)-8:2, N(2):O(2)=7:3, N(2):O(2)=6:4) at a constant heating rate of 20 degrees C/min. The effect of oxygen concentration on the pattern of combustion was analyzed. The experimental results showed that the ignition and burnout temperatures of biomass and semi-coke decreased with increasing oxygen concentrations. Aside from the weight loss during moisture evaporation, the combustion of individual biomass and semi-coke samples took place in two stages. In the first stage, the release and combustion of volatiles took place, while in the second stage, the combustion of fixed carbon occurred. The combustion of blends took place in three stages (again aside from moisture evaporation) corresponding to the sum of the individual stages of combustion of biomass and semi-coke. Several combustion reaction kinetics mechanisms were tested using the Coats Redfern Method in order to find the mechanisms responsible for sample combustion. The activation energy was determined assuming that single separate reactions occur in each stage of thermal conversion. The results showed that a first-order chemical reaction model provided the best characterization of the first stage of biomass oxidation and oil shale semi-coke combustion. However, diffusion was found to be responsible for the second stage of biomass and semi-coke combustion. For blends, a first-order chemical reaction provided the best model for the first and second stages of combustion whereas a diffusion mechanism was the best for the third stage.