Chitosans with chemical composition ranging from a fraction of N-acetylated units (F(A)) of 0.01 to 0.61 were used to prepare fluorescence labelled chitosans by reductive amination with 9-anthraldehyde. Fluorescent chitosans with a low theoretical degree of substitution (DS, 0.001-1%) were prepared, and the actual DS of the products were determined by UV and (1)H NMR spectroscopy. The fluorescence excitation and emission spectra of the chitosan with F(A) of 0.09 and DS 1% showed an excitation maximum at 254 nm and an emission maximum at 413 nm. The intrinsic viscosities ([eta]) of the fluorescent chitosans were compared to those of the original chitosans, showing that the derivatisation procedure lead only to a negligible decrease in [eta]. The conformation of these fluorescent chitosans with very low DS-values is not altered and they can conveniently be directly quantified by UV or fluorescence spectroscopy.