A new approach for improving fire resistance of cellulosic materials is provided, especially when the cellulosic material is to be used in polymer composites. Cellulosic material is treated with an aqueous mixture of alkali metal or ammonium hydroxide and alkaline earth or aluminum metal salt simultaneously with or within a short period of time of preparing the mixture. The treated cellulosic material becomes self-extinguishing and may also have improved thermal stability, improved interfacial thermal resistance, improved resistance to damage by oxidants and other chemical agents, improved resistance to biological agents and/or improved resistance to damage by ultra-violet light.