The adsorption of several chitosan samples with different degrees of deacetylation (65, 73, 85, and 92%) and their interactions with the main components of reed pulp suspensions were investigated with gel permeation chromatography, phenol/sulfuric acid spectrophotometry, microelctrophoresis, and retention/drainage methods. The results showed that the chitosan additive was almost completely adsorbed onto the surfaces of the cellulosic fibers, especially onto the surfaces of fines in a variety of cellulosic systems at low dosages corresponding to those used in industrial operations. This adsorption increased as the degree of deacetylation of chitosan increased. Moreover, the aggregation of the fine cellulosic particles was maximum at a dosage of about 10 mg/kg of chitosan. The uptake of dissolved and colloidal carbohydrates always occurred in the cellulose suspensions upon the addition of chitosan. These phenomena were related to the maximum retention of fines and drainage performance of the pulp suspensions; the optimum drainage dosage was found to be about 5-8 mg of chitosan per gram of oven-dried pulp. The interactions between chitosan and the cellulosic substrates were dominated by a bridging mechanism at pH approximate to 7 for these experiments. (C) 2003 Wiley Periodicals, Inc.