Abstractp–p stacking of the aromatic groups in SL. Results show that the charge-free aromatic groups of SL tend to form oriented p–p stacking with the spectroscopic characteristics of J–aggregates. The formation of J–aggregates in SL are recognized by a significant spectral red shift in fluorescent excitation spectra. The other effects that may cause spectral shift, such as the SL species, solvent effect, and the impurities, are investigated to confirm that the formation of J-aggregates is the only viable explanation for the significant spectral redshift of SL. causes molecular shrinkage of SL polyelectrolytes, but has no influence on J–aggregates of the aromatic groups as detected by lack of spectral shift, indicating that the aromatic groups are charge-free. This suggests that not all the aromatic groups but only the charge-free aromatic groups can form p–p stacking. This work demonstrates the presence of J–aggregation in aqueous SL solutions for the first time, which gives an insight in understanding the preferred orientation of the aromatic groups in lignin-based biopolymers.