The particle size distribution of different sludges from sewage treatment plants was compared. Since the dewaterability of a sludge depends on its particle size distribution, this property can be used to characterize dewaterability. Thus, as shown by an example, the change in the dewaterability by conditioning a sludge with a polymer reflects the change of the particle size distribution. The measurement of the particle size distribution at a defined position in a settling tube during the settling process may also be used to study the settling behaviour of a sludge. The floc structure of sludges from different treatments varies in a wide range. It is not possible, therefore, to give a full characterization by the particle size distribution only. Furthermore, structural elements of the sludge flocs cannot be neglected. Taking this into consideration, new dewatering parameters have to be defined in order to characterize dewaterability as a function of the particle size distribution.