The effect of the contact time between cationic polymers (polyacrylamides and cationic starches) and stock on retention and dewatering has been studied using a new type of equipment. Dewatering is carried out under low pressure and the addition of chemicals and other operations are controlled by a microprocessor. The retention often shows a maximum after a polymer contact time of 10-20 s. It is found that long contact times drastically reduce the effect of the added polymer. This is explained as being due to floc rupture, degradation of the polymer and poor reflocculation. The dewatering time is extremely sensitive to the polymer contact time and increases steadily with increasing contact time.