The process streams in the forest industry contain a large amount of hemicelluloses that today ends up in the wastewater. This is an unfortunate loss of a renewable raw material. The hemicelluloses can be isolated from the process stream by using membrane filtration in a process that produces purified water as a by-product, thereby facilitating increased recirculation. However, process streams from the forest industry contain both aromatic compounds and inorganic ions that are known to cause fouling of the membranes. Thus, the most suitable membrane and pretreatment from a cost-efficient point of view must be applied to avoid fouling and life-time shortening of the membranes during operation. In the present investigation, fouling during ultrafiltration of a process stream from the production of Masonite was studied. The fouling of a hydrophilic membrane made of regenerated cellulose was less severe than the fouling of hydrophobic membranes made of polyether sulphone and a composite fluoropolymer. Pretreatment of the wastewater with activated carbon resulted in higher flux and less fouling of both hydrophilic and hydrophobic membranes. Lowering the pH of the wastewater led to less severe fouling of the regenerated cellulose membrane, but lower flux of the composite fluoropolymer membrane.