Lanthanum-139 n.m.r. spectra are reported for a variety of lanthanum complexes and systems. The 139La linewidths vary greatly and, as expected, an important factor appears to be the symmetry of the environment around the lanthanum. Information concerning complex formation by the La3+ ion is obtained from the variations in both the 139La chemical shifts and the linewidths when a ligand is progressively added to anhydrous La[ClO4]3 in MeCN. Slow ligand exchange is found with the [La{P(NMe2)3O}6]3+ ion, and with the ethylenediamine and diethylenetriamine complexes.