Fourier transform infrared (FTIR) spectroscopy was used to study the chemical changes of masson pine (pinus massoniana lamb.) decayed by the brown-rot fungus Wolfiporia cocos (Schwein. ) Ryvarden & Gilbn. for different durations up to 23 weeks. The ratios of height of the lignin/holocellulose and holocellulose/lignin IR peaks were measured, and the klason lignin content and holocellulose content of the sound wood and brown-rotted wood with different level of decay were analyzed by wet chemical methods. The relationship between the two chemical components and the ratios of IR peak height was also established. FTIR spectra showed that, during the first 15 weeks of decay, the intensity of absorption bands at 1 736 and 1 372 cm-1 ascribed to holocellulose decreased gradually, accompanied by a successive increase in the intensity of band at 1 510 and 1 225 cm-1 ascribed to lignin. However, the intensities of holocellulose bands at 1 736 and 1 372 cm-1 had a little increase, and the intensities of lignin bands at 1 510 and 1 225 cm-1 had a very slight decrease after 15 weeks of decay. There was a very good correlation between the ratios of height of the lignin/holocellulose (I1 510/I1 736, I1 510/I1 372, I1 225/I1 736 and I1 225/I1 372)and the klason lignin content or holocellulose content. The coefficients of determination for the klason lignin content and the holocellulose content were 0.97-0.99 and 0.96-0.97, respectively. High coeffieients of determination were also obtained between the holocellulose/lignin peak height ratios and the holocellulose content(R2＝0.96). The above results suggest that, in the system studied, the klason lignin content and holocellulose content of wood decayed to differnent levels could be determined with reasonable accuracy by the FTIR technology.