The performance of five polymeric nanofiltration and tight ultrafiltration membranes was investigated during separation of hemicellulose from process liquors containing 200 g/l sodium hydroxide. The effect of pressure and temperature on flux and retention was determined to enable optimization of operating conditions. Retention behavior was further elucidated by calculating retention as a function of molar mass obtained from size exclusion chromatography (SEC) between 100 and 50,000 g/mol. The experimental data show that hemicellulose is almost quantitatively retained at molar masses above 1,000 g/mol. Two of the membranes exhibiting excellent overall hemicellulose retention (around 90%) are most suitable for efficient removal of hemicellulose from the process. The results emphasize the need for accurate retention data from industrial effluents and process conditions.