Causing the decreasing of the surface hardness and of the mechanical characteristics as strength, plasticity, toughness and especially fatigue, the phenomenon of internal oxidation occurred during carburising of steels alloyed with chromium, titanium, silicon, manganese etc request a particular attention. In the presence of alloying elements with high affinity for the oxygen dissolved in the metallic matrix and for a diffusion coefficient of oxygen along austenitic grain boundaries increased by several orders of magnitude due to vacancies in excess, the premises for the appearance in these areas of certain oxide films with intergranular disposal were created. This phenomenon occurs more intensively in the case of atmospheres with high carbon monoxide, produced directly in workspace of heat treatment equipment, as is the case of the atmospheres produced by instillation and pyrolysis of organic liquids. The paper reviewed the general aspects of the phenomenon of internal oxidation during carburizing of alloy steels in gaseous media, with particularization for carburising media obtained through instillation and pyrolysis of organic liquids.