Nanofiltration of process liquors from viscose-type textile fibre production was investigated using a laboratoryscale crossflow test apparatus. Feed solutions contained about 200 g/l sodium hydroxide and a high concentration of hemicellulose as a contaminant originating from the raw material wood. The effects of various pretreatments and the addition of reagents on flux, retention and irreversible fouling were studied. Reagent addition affected the solution state of hemicellulose thus purposely provoking aggregate formation. Dynamic light scattering and turbidity measurements were used to track the evolution of hemicellulose aggregates. Aggregation generally coincided with distinct flux decline during nanofiltration thus indicating that this phenomenon may result in the build-up of gel layers at the membrane surface. Partial neutralisation with sulfuric acid caused extremely severe fouling, whereas methanol addition had a minor effect. The impact of heat treatment and oxidative degradation of hemicellulose on nanofiltration performance was attributed to changes in molar mass distribution.