The effect of cationic polymers (cationic corn and potato starches, polyacrylamide and fractionated modified polyethylene imine) on the water retention value (WRV) of different pulps (bleached chemithermomechanical pulp (CTMP), bleached kraft pulp and unbleached sulfide pulp) was investigated. The pulps were in hydrogen ion state. It was found that for for well refined CTMP and processed pulps the WRV decreases upon polymer addition. Decreases of up to 10-20% were observed. The reduction in the WRV is explained in terms of the penetration of cationic polymers into the surface region of the fiber wall where the polymer neutralizes ionized surface groups, and of an aggregation of fibrils located on the external fiber surface. The penetration mechanism is supported by the fact that increases in electrophoretic mobility caused by polymer adsorption decrease with time for those pulps which show a decrease in WRV upon polymer addition. Such a mobility decraease is not found for the low refined CTMP and in this pulp the WRV does not decrease.