The effects of temperature, concentration of caustic soda and liquid to solid ratio on hydrothermal treatment of perlite with caustic solution to produce metal silicate solutions were investigated. The reaction was carried out in laboratory autoclave of 1 L capacity over a range of experimental conditions. The treatment time was kept constant at I hour. The chemical compositions of liquid products were analyzed by standard wet chemical methods. The results indicated that, under favorable conditions, aqueous metal silicate solutions with a high yield of about 80 percent, and having SiO2/Na2O molar ratios of 0.6 to 1 can be produced by this method. (C) 2001 Elsevier Science Ltd. All rights reserved.