Carboxylic acid bound to xylans in the fibre matrix cause a pH gradient between the fibre and the surrounding solution, known as the Donnan effect. The gradient is dependent on the ionic strength of the fibre solution. When meta1-free kraft pulp was used as a substrate for the Trichoderma reesei xylanase, the apparent pH optimum of the xylanase at low ionic strengths was found to be significantly higher, around 9, when a monovalent hydroxide was used for pH adjustment, as compared to the pH optimum obtained with a divalent metal hydroxide, i.e. pH 5–6. By increasing the ionic strength of the pulp solution by salt addition, the pH optimae of the xylanase became 5–6 with all the hydroxides. This difference was caused by the Donnan effect. Thus, at low ionic strengths the actual pH is lower than that measured in the solution, resulting in an increased apparent pH optimum of the xylanase. In practice, these results are important when applying xylanases on practical, fibre bound substrates, such as kraft pulps.