Abstract: Radionuclides are immobilized in virtually an insoluble form by reacting at a temperature of at least 90.degree.C as an aqueous alkaline mixture having a solution pH of at least 10, a source of silicon, the radionuclide waste and a metal cation, the molar ratio of silicon to said metal cation being on the order of unity to produce a gel from which complex metalosilicates crystallize to entrap the radionuclides within the resultant condensed crystal lattice. The product is a silicious stone-like material which is virtually insoluble and non-leachable in alkaline or neutral environment. One embodiment provides for the formation of the complex metalo-silicates, such as synthetic pollucite, by gel formation with subsequent calcination to the solid product; another embodiment utilizes a hydrothermal process, either above ground or deep within basalt caverns, at greater than atmospheric pressures and a temperature between 90.degree. - 500.degree.C to form complex metalo-silicates, such as strontium aluminosilicate.