The exact nature of extra-framework aluminium is not clear, but it has been shown to consist of four-, five-, and six-coordinated aluminium species.Furthermore, it has been suggested that extra-framework aluminium can be cationic and can replace protons in bridging hydroxyl groups. The hydrothermal stability of a zeolite indicates how prone the framework is to dealumination under hydrothermal conditions. Therefore, a zeolite that does not undergo dealumination and retains acid sites during treatment by steam exhibits high hydrothermal stability. The hydrothermal stability of a zeolite depends on the number of aluminium atoms in the lattice, the framework type, and the type of counter-cations. Increases in temperature, the volume of steam, or the duration of hydrothermal treatment all lead to more severe dealumination.