The typical reactive structure stabilized in a diffusion layer in standard conditions can be significantly modified whether injected flows are diluted and/or preheated. The flow high initial enthalpy and the low fuel and/or oxygen concentration can drastically modify the structure of the oxidative and pyrolytic region due to change of the physical and chemical kinetics respect to conventional diffusion flame. Such operative conditions are typical of MILD (Moderate or Intense Low-oxygen Dilution) combustion processes. The effect of inlet conditions on the stabilized reactive structure has been studied by analyzing the behavior of a steady, one-dimensional diffusive layer. The change of the structures of the reactive region induced by a hot oxidant and diluted oxidant flow (HODO) fed towards a fuel jet at environmental temperature was numerically analyzed by means of temperature and heat release profiles, that are key parameters to understand the main features of the reactive region. In addition, the effect of diluent nature was studied by comparing the reactive structures also obtained with steam and carbon dioxide.