Quantitative structural studies of adsorption structures, especially using scanned-energy mode photoelectron diffraction (PhD) and low energy electron diffraction (LEED), are now capable of providing interatomic bondlengths with sufficient precision to provide chemically significant information regarding the nature of adsorbate-substrate bonding. Selected examples of such studies of molecular adsorbates on mainly metal surfaces are presented which provide insight into the relationship between adsorption energies, coordination site and both adsorbate-substrate and intramolecular bond lengths. Specific examples include simple hydrocarbons on transition and noble metal surfaces, and pure and coadsorbed CO on Ni and NiO surfaces. (C) 2004 Elsevier B.V. All rights reserved.