The release behavior of sodium (Na) from sawdust during air and oxy-fuel combustion was investigated based on temporal detection, thermodynamics and kinetic study. The results indicated that the Na release behavior was significantly affected by the reaction temperature and combustion atmosphere. On the whole, the total release percentage of sodium in oxy-fuel environment was inferior to that in air combustion. The apparent kinetic parameters of Na release during two combustion processes were derived from the experiment results of time-resolved Na concentration and the inverse method. For the apparent kinetic parameters of Na release, the activation energy in oxy-fuel combustion is larger than that in air combustion. Besides, the effect of chlorine on Na release was investigated based on the continuous temporal detection system, and the promoting effect revealed a significant dissimilarity during air and oxy-fuel combustion. The promoting effect of chlorine on sodium release in oxy-fuel environment was much weaker than that in conventional air combustion, which may be mainly due to the variations of chlorine speciation in the presence of mass CO2. Thermodynamic equilibrium analysis was also carried out to identify the possible sodium species during air and oxy-fuel combustion processes. The influence of large excess of CO2 in oxy-fuel environment on the sodium release paths was proposed and summarized by means of a route map.