[en] The phase diagrams of adsorbed monolayers of D₂, H₂, O₂ and ³He on graphite have been measured by neutron diffraction. H₂ and D₂-layers have a registered √3 structure at low coverages, and at monolayer completion they have a dense triangular structure, which is incommensurate with the substrate. Between the two densities there is a broad region of coverages where the structure is triangular but the lattice parameter varies with coverage. Adsorbed O₂ layers are found to have three distinct phases which are all incommensurate with the substrate lattice. Due to the large absorption cross section of ³He the neutron diffraction measurements from the ³He layers are much more difficult, but both the registered √3 structure and the dense triangular structure near monolayer completion have been observed. Inelastic neutron scattering results are presented for adsorbed H₂ and D₂ layers