The adsorption of the protein fibronectin onto smooth metal oxide surfaces has been monitored in situ and in real time at both low and high ionic strengths using optical waveguide lightmode spectroscopy (OWLS). The kinetics of the evolution of the adsorbed layer thickness, its packing density, and of the total amount deposited were analyzed and used to deduce that at low ionic strength the protein has a compact conformation prone to lateral clustering at the surface, and at high ionic strength it is in a random extended conformation.