Analysing gaseous nitrogen oxides emissions at the combustion system outlet (exhaust pipe, central heating, glass kiln,...) are a current environmental topic. Yet, studying these pollutant involves two difficulties : first a great number of NOx do exist, and secondly the system can shift quickly. The objective of this work consists of examining results of literature and studying more specially NO/NO 2/O 2 system to clarify these problems. For this purpose theoretical and experimental works concerning kinetics and thermodynamics of NOx transformations have been carried out. The NOx maximum starting concentration reaches 1000 ppm in an oxygen-containing atmosphere and with temperature comprised between 25 $^{\circ}$C and 1000 $^{\circ}$C. These parameters are representative of real combustion conditions. An apparatus has been built to determine both experimental equilibrium and kinetic constants for the reaction 2 NO + O 2 = 2 NO 2. Rate constants have been measured for temperature lower than 300 $^{\circ}$C. Above 300 $^{\circ}$C, the catalytic influence of the reactor walls has been clearly established. For each temperature, NOx concentrations at equilibrium give experimental thermodynamic data, in agreement with the theoretical approach.