Beech wood boards were studied with a cone calorimeter. As the specific density of these materials is greater, the total amounts of CO and CO, produced were bigger than values measured on spruce specimens. Also, the shape of the heat release rate (HRR) curve was different as its second peak was bigger than the one observed on spruce wood. It was shown that impregnation of wood with water solutions of H3BO3 resulted in decreased HRR values with increasing acid concentrations. Mass loss and CO and CO, production rates also decrease in the same way. Impregnation with water-soluble NaOH-silica resulted in improved data, when compared to untreated specimens, but were not better than those obtained with H3BO3. Simple mixing of both fire retardants gave no improvement, nor was the two-step impregnation with drying effective after the first step. When the second step-treatment with acid was followed without drying after impregnation with NaOH-silica in the first step, then improvement was evident. This is explained by precipitation of the silica content inside the material when the NaOH was neutralized with H3BO3, which could penetrate inside when the material was not dried before the second step. The improvement resulted in lower smoke production and longer times to ignition. (C) 2005 Wiley Periodicals, Inc.