Atmosphere is one of the most significant factors in the thermal decomposition of biomass. In domestic or industrial biomass boilers, ambient oxygen concentration varies along the time which means that the reaction will change from pyrolysis to combustion. In this way, to analyze and compare each thermochemical conversion processes a simple analytical method, the non-isothermal thermogravimetric analysis, is carried out under oxidative (air) and non-oxidative (argon) environments at 10 °C/min and as a function of different flow rates (2 to 150 mL/min). Additionally, this work was complemented by a kinetic analysis considering a first-order reaction to each conversion stage and using the Coats-Redfern method. The effect of the atmosphere on the thermal decomposition behavior was evident. It was observed that the thermal decomposition of pine wood particles varied from three to two stages when the oxidative or inert atmosphere was applied. The presence of oxygen changes the mass loss curve mainly at high temperature, around 350 °C, where char reacts with oxygen. The maximum mass loss rate from experiments with the oxidative atmosphere is 15% higher than in an inert atmosphere, the average char combustion rate is approximately 5 times higher and the heat released reaches 3.44 times higher than in an inert atmosphere. Ignition and combustion indexes were also defined and its results revealed that particles are ignited faster under oxidative atmosphere and, on average the combustion index is 1.7 times higher which reinforces the more vigorously way that the samples are burned and how faster char is burned out in the experiments with air. Regarding the kinetics analysis, higher activation energies and, consequently, lower reactivity was obtained under the oxidative atmosphere for the second stage (~125 kJ/mol) and under the inert atmosphere for the third thermal conversion stage(~190 kJ/mol).