Two luminescent terbium and europium lanthanide chelates were efficiently embedded into silica nanoparticles by using a reverse microemulsion process. The incorporation was achieved without covalent bonding between the lanthanide chelates and the silica matrix. To investigate the efficiency of the incorporation process and the stability of the silica-encapsulated lanthanide complex, a method based on a radioactive probe was developed; γ-emitting europium (152) chelates were synthesized and incorporated into silica nanoparticles. Measurements of the γ activity through the entire synthesis allowed the accurate characterization of the incorporation efficiency of the used chelates. A clear correlation was established between the physicochemical properties of the different chelates and the measured incorporation efficiencies. A very efficient noncovalent incorporation of lanthanide chelates in highly stable nanoparticles was achieved by tuning the chelate properties, thus rendering the development of lanthanide-based fluorescent nanoparticles easier. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.