Fourier transform infrared (FTIR) spectroscopy was used to examine lignin modification in Pinus sylvestris L. (Scots pine) and Fagus sylvatica L. (beech) decayed to different weight losses by the brown-rot fungus Coniophora puteana. Samples of different weight losses, ranging from 3 to 64% for beech and 15 to 62% for pine, were analyzed along with undecayed controls. The ratios of the heights of the lignin/carbohydrate FTIR peaks were determined, and the lignin contents of the blocks were measured with the acetyl bromide method. The ratios of the reference peaks for lignin against polysacchalide (i.e., cellulose and hemicellulose) peaks were compared with the lignin content of the wood determined by the acetyl bromide method. A good correlation was obtained for ratios of some of the lignin/carbohydrate reference peaks (lignin peaks at 1505 and 1330 cm(-1) for beech and at 1511 and 1225 cm(-1) for pine against polysaccharide peaks at 1734, 1375, and 1158 cm(-1)) against the lignin content for both wood species decayed to different levels. (C) 2004 Wiley Periodicals, Inc.