The composition of the dissolved and colloidal fractions of a “model” white water prepared from a spruce-pine-fir/hemlock thermomechanical pulp was determined. The impact of these fractions on paper properties was assessed and the ability of enzymes to degrade the different components was investigated. The colloidal particles in the white water had an average size of 0.5 μm and a size range from 0.1 μm to 2 μm. Lignins, resin and fatty acids, and esterified extractives, such as sterol esters and triglycerides, were the main constituents of the colloidal particles, while the lignans and neutral polysaccharides were predominantly dissolved in the white water. Reductions in paper strength were mainly caused by the dissolved substances, whereas the colloidal substances were primarily responsible for the reduction in paper porosity and optical properties. Added laccases were able to degrade most of the extractives while lipases specifically hydrolyzed esterified extractives present in the colloidal fraction.