Recent neutron-diffraction studies on dilute solutions have shown that a change in coordination numbers from nine to eight water molecules in the first hydration sphere occurs when going from the light to the heavy lanthanide (III) ions. To acquire a deeper insight into the structure of the first coordination sphere of these ions Monte Carlo and molecular-dynamics computer simulations were performed. The initial results will be compared with neutron-diffraction data and solid-state structures. Recently there has been increasing interest in Ln3+ complexes as relaxation agents in magnetic-resonance imaging. We have therefore studied water exchange on a series of gadolinium (III) complexes by oxygen-17 NMR. Results will be given and compared with octaaqua Gd3+.