In accordance with the changes in the free energies of formation of hydrocarbons as a function of temperature, methane is unstable in terms of its elements from 530 degrees C, but remains the most stable of hydrocarbons up to 1030 degrees C. Using methane, it is necessary to reach temperatures of 1200 and 1300 degrees C to produce, respectively, acetylene and ethylene. However, since acetylene becomes the most stable hydrocarbon from 1230 degrees C, it is acetylene that predominates sit this temperature. According to the variation in the enthalpies of formation as a function of temperature, the pyrolysis of methane demands a very high energy input to form primarily acetylene. We calculated the composition of a mixture CH4, C2H4, C2H2, C6H6 and H-2 at thermodynamic equilibrium by minimizing the Gibbs energy of this system. This method does not require prior knowledge of the chemical reactions taking place at equilibrium. The calculation parameters are the initial H/C ratio, the temperature, the pressure and the Gibbs energies of each substance. The analysis of the complex chemical equilibria helps to identify temperature zones corresponding to stability domains of certain molecules. Below 1200 degrees C, the disappearance of methane is slight, and the main hydrocarbon produced is benzene, followed by ethylene, without any significant formation of acetylene. Species with acetylenic structures (like C2H2) appear above 1200 degrees C, as well as C2H, C3H and radicals such as H and CH3. This means that the C-C and C-H bonds split above 1200 degrees C. The species present have an increasingly small H/C ratio, and the initial hydrogen is found virtually in molecular form. The presence of hydrogen in the reaction medium has the effect of increasing the proportion of the hydrogen-rich species, chiefly CH4 and C2H4, and of decreasing the conversion of methane.