In this paper, we will discuss the criteria of fuels characterization. For hydrocarbons, these criteria depend on the relation of the hydrogen (H) and carbon (C) contents of the respective hydrocarbons. To analyze from the energy-Lecologic point of view a large range of fuels (coal, refinery residues etc.), we conclude that the best criterion is the pollution indicator Pi(g). Also, we will classify the fuels from the energy-ecologic point of view.