The content of bound phenolic substances in flax fibres was determined by 13C CP-MAS solid-state NMR spectroscopy, a novel method that had not previously been applied to plant materials of very low lignin content. Van Soest (modified Klason) lignin and acetyl bromide lignin were also determined for comparison. The 13C NMR spectra were obtained at low field (25 MHz) under conditions that had been shown, by comparison with single-pulse excitation, to give quantitative responses from the aromatic carbon atoms of wood lignin. The Van Soest lignin content was 3.2% and the acetyl bromide lignin content was 2.4%. The solid-state 13C NMR spectra showed that these values were considerably overestimated and that the quantity of aromatic carbon present corresponded to a lignin content of 0.9%. However, the aromatic material present was not lignin but appeared to be predominantly an anthocyanin of the cyanidin type. These results illustrate the deficiencies of the accepted methods of determining lignin at low levels, and the potential of solid-state 13C NMR for this purpose.