Hornification is the loss of swelling of the fibre wall resulting from a drying-and-rewetting cycle: it is associated with a stiffening of the fibres which reduces their ability to form inter-fibre bonds. In this paper,factors affecting the extent of hornification are examined using the solute exclusion technique to measure swelling. Two major observations are that hornification is a feature of low-yield pulps and that it is primarily brought about by the removal of water from fibre walls rather than anyassociated heattreatments. The mechanism is proposed to be an increase in the degree of cross-linking between microfibrils due to additional hydrogen bonds formed during drying and not broken during rewetting. In high-yieldpulps, the presence of lignin and hemicellulose between the microfibrils prevents this bonding. In low-yield pulps, the absence of these materials permits hornification to occur. However, hornification can be prevented by interfering with hydrogen bond formation either by i), partially substituting the cellulose hydroxyls by groups that do not hydrogen bond, or ii), drying pulps in the presence of additives that”bulk” the fibre wall and so effectively replace the ligno-hemicellulose gel.