Abstract: A semi-quantitative study of willow bark dye adsorption on two different cellulose materials using biomordants was carried out. The studied celluloses were microcrystalline cellulose (MCC) AaltoCell and regenerated Ioncell-F (IC) fibres. The dye was a hot water extract of willow bark and the adsorption to cellulose was carried out using carboxylic acid-containing biomordants, namely, oxalic acid, citric acid and tannic acid. Alum was employed as the reference mordant. A semi-quantitative estimation of the dye uptake was conducted using high-performance liquid chromatography equipped with a diode array detector and also by visual inspection, as well as an evaluation of the coloration using CIELab parameters. The mechanism of the dye adsorption on the cellulose surfaces was studied via Fourier Transform–infrared spectroscopy. According to the results, MCC had a higher affinity for polyphenolic dye than the regenerated cellulose fibres. Dye uptake on MCC was 50%-80% and 44%-57% on IC. For MCC, the biomordants improved the dye uptake more effectively than the control mordant, alum, whereas for IC the biomordants were less effective than alum.