White pinewood was impregnated with thiourea-formaldehyde (TUF) resin and orthophosphoric acid (OPA) as a crosslinking agent. The best weight gains (55-138%) were obtained after impregnation with an aqueous solution of TUF resin for 1 h and impregnation with aqueous solutions of OPA at different concentrations for 1 h. Water uptake of treated wood was found to be 23% after a water-soaking test of 168 h, and a maximum antiswell efficiency was found to be 18% for a 85% OPA solution. Compression strength of the treated wood also improved with the highest value 62 KN/m(2) for wood treated with a 70% OPA solution versus a value of 40 KN/m(2) for untreated samples. Fire retardancy of the treated wood samples was also improved based on concentration of OPA.