The objective was to define the nature of the plant fraction identified as lignin by various procedures. Seven materials (alfalfa, orchardgrass, red clover, and switchgrass hays, a mixed hay, peanut hulls and wheat straw) were examined. The following lignin determination procedures were studied: 72% sulphuric acid, permanganate, chlorite and triethylene glycol, using a variety of combinations of fibre source (acid detergent, hot water/organic solvent and 0.5% ammonium oxalate extracted) and lignin determination procedures. Fractions were examined by infrared spectroscopy in an effort to determine the compositional changes induced by the various chemical procedures. Results showed large differences in the spectra obtained by the various procedures and indicated severe problems for the various lignin assays. The most common problem was the tendency for at least some crude protein to be included as part of the lignin value. The chlorite and triethylene glycol methods for lignin determination did not appear to remove all the lignin present, and there was some indication of carbohydrate extraction by the various procedures. Sulphuric acid showed retention of protein and perhaps carbohydrate. Finally, the problems varied both with the assay in question and with the material being assayed.