Most of the energy generated during the fission process is released as kinetic energy of the fission products. This energy moves the fission products a distance of a few microns in solid materials. When the fissionable material is prepared as a powder of particles with diameters smaller than the range of the fission products in the material used, it is expected that the fission products will leave the particles of the fissionable material. To avoid the penetration of the fission product into an adjacent particle of fissionable matter, the latter may be diluted with a liquid or solid diluent. The use of solid diluents having strong adsorption properties is believed to improve the separation between fission products and fuel when sedimentation in water is chosen as the separation method. In a series of experiments, mixtures of U3O8  with infusorial earth and silica gel as diluents having strong adsorbing properties were irradiated. About 95% of the fission products were found in the diluent. Most of the activity of the U3O8 was due to Np. The readsorption of fission products to U3O8 was smaller than in previous experiments in which no adsorbent was mixed with the fissionable material. Surface activation of the U3O8 was found after irradiation. About half of the fission products taken up by the diluent were found to be adsorbed at its surface. Mean fission-product ranges in U3O8 were estimated on an experimental and theoretical basis and agreement between theory and experiment is found to be good for most of the fission products.