The aim of this study was to increase the resistance of fire retardant chemicals for leaching in order to improve the fire resistance of wood. For this purpose, Scots pine (Pinus sylvestris L.) and Chestnut (Castanea sativa Mill.) wood impregnated with 3 % aqueous solutions of boric acid, zinc chloride and aluminum chloride at 60 C by using vacuum- press impregnating method according to ASTM D1413-07 standard. In order to increase leaching resistance of the chemicals, wood samples post-treated with sodium silicate and then sodium bicarbonate to form insoluble silica gel in wood. After leaching test, it was determined that silica gel treatment decreased the amount of leachant in all three impregnation solutions. Effect of silica gel on the thermal behavior of treated samples was determined via termogravimetric analysis (TGA). According to the results of TGA silica gel treatment improved the fire resistance property of wood especially in boric acid treated wood.