THERMOPTIM defines three categories of substances: pure ideal gases, ideal gas compounds, and condensable vapors (which are pure). The perfect gas correspond to the particular case of ideal gases whose specific heat is independent of temperature. The substance may be pure, in which case its properties are predefined in the package, or compound. In this case (which is only possible for a gas), the user must define the composition from the other gases present in the database, indicating for each, its name and its mole or mass fraction. The properties of the compound substance are then determined from those of its constituents.