Abstract: The invention relates to a process for enhancing the concentration of the fiber derived dissolved and colloidal substances in process water that comes from mechanical pulping. According to the invention, process water is treated with enzyme preparations that have an effect on carbohydrates and that contain mannanase and/or endoglucanase, in particular, which break down the soluble carbohydrate polymers contained by the process water into low molecular weight oligosaccharides. The treatment prevents an increase in viscosity so that process water can be concentrated into a high dry content, or the treatment can be used to lower the viscosity of previously concentrated process water to facilitate after-treatment.