The synthesis and characterization of new radiolabeled Se-75-silica nanoparticles are described. These particles have been prepared by dispersion polymerization of tetraethylorthosilicate in ethanol in presence of radiolabeled (75)selenium-colloids. The amount of Se-colloids encapsulated within the silica matrix was then increased by two successive seeded polymerizations of tetraethylorthosilicate on the previously formed particles. Kinetic studies and the effect of various factors, i.e. sodium selenite, water and ammonium hydroxide concentrations, on the diameter of these nanoparticles have been elucidated. The Se-silica nanoparticles have been covalently coated by the self-assembled technique with a variety of omega-functionalized alkylsilane compounds. Characterization of these coatings has been performed by Fourier transform-infrared spectroscopy, elemental analysis and floatability studies. The potential use of these varieties of omega-functionalized radiolabeled nanoparticles for drug delivery purposes has been demonstrated in a separate article.