papers, mainly for printing and writing or converting purposes. Internal sizing can be obtained through the addition of alkyl ketene dimer (AKD) into the pulp suspension. It is well known that AKD sizing efficiency depends on process parameters such as pulp grade, dissolved and colloidal substances, and paper drying and storage conditions, but the mechanisms involved are not totally understood. This study focused on the effect of wood extractives on sizing efficiency. First, a thermomechanical pulp (TMP) was used before and after extraction with various solvents and its response to sizing was evaluated. Then, dehydroabietic acid (DH) and hemicelluloses were added to a bleached kraft pulp (BKP) and their effect on sizing, alone or with the coupled action of calcium ions, was studied. The results obtained confirmed that TMP is more difficult to size than BKP and that extraction performed on TMP allows retaining an acceptable sizing level. DH and hemicelluloses impair AKD sizing efficiency and they decrease both retention and reaction of AKD particles onto the fibres. Finally, calcium ions, when alone, may negatively impact AKD retention, but they prevent in certain conditions the detrimental action of DH and xylan. (C) Koninklijke Brill NV, Leiden, 2011