Two-dimensional phase diagrams of adsorbed phases are usually determined by adsorption isotherm measurements that infer the existence of two-dimensional (2D) gases, liquids and solids. It is shown how the nature of these various phases, particularly 2D liquids and 2D solids, can be determined by electron or neutron scattering. We pay here a special attention to the phase transitions between 2D condensed phases, i.e. 2D melting and 2D polymorphism. © 1979.