Abstract: The organic chemical waste content of contaminated materials such as soils and debris is immobilized by mixing such wastes with an agent consisting of a mixture of particulate rubber and biogenic amorphous silica. Where the semi-volatile content of the waste is known to predominate a greater portion of particulate rubber is advantageous and where such waste is known to be entirely semi-volatile organic material such agent advantageously consists essentially or comprises particulate rubber. Conversely, where the organic content of such wastes is predominately volatile organic material a greater portion of biogenic amorphous silica should be employed and where such organic waste is known to be entirely volatile organic such additive is advantageously biogenic amorphous silica.