An aqueous colloidal dispersion of at least one lanthanide compound and a complexing agent with a pK (cologarithm of the dissociation constant of the complex formed by the complexing agent and the lanthanide cation) or more than 2.5. The dispersion can also include a compound of at least one other element selected from groups IVa, Va, VIIa, VIIa, VIII, Ib and IIb of the periodic table. A process for preparing the dispersion starts by forming an aqueous mixture comprising at least one lanthanide salt, optionally a salt of one of the elements mentioned, and the complexing agent. A base is added to the mixture, which is heated to obtain the dispersion. Methods that use the dispersion are also disclosed.