Immiscibilities between silica and lithium disilicate were investigated. Alpha particles ejected from 6Li atoms by thermal neutrons were used to autoradiograph surfaces representing diffusion-zone cross-sections. Etchant-enlarged alpha tracks in nitrocellulose disclosed the distribution of lithium. For autoradiography, procedural details were established, suitable operating conditions were selected, and the method was calibrated. For diffusion, adequate couples had to be prepared. Subsequent experiments indicated phase separations along an 885° C isotherm of the silica-lithium disilicate system. There was evidence of metastable compositions near 30, 25, 20, 16, 11, and 7 mol % of lithia in silica. These results have been given a very tentative interpretation: The first two compositions may correspond to metastable extensions of liquidus boundaries, projected through a eutectic point of the system. The third and sixth compositions are probably points on a liquid-liquid miscibility boundary, and the fourth and fifth compositions could be on an associated spinodal boundary.