The paper describes the specific features of flow processes. The literature assumes that the separation criterion in flow processes is the diversification of settling velocities of particles in the separation medium. Nonetheless, it is very common to identify the realization of many flow processes with particle classification (sizing), i.e. separation according to the particle size, or cleaning (sorting), that is separation according to the particle density. The reason behind this, is the lack of a method for characterizing the particle populations according to the characteristic which is assumed as the separation criterion, here, the settling velocity of particles. The basic methods for characterizing the particle population and their relationship with partition curves have been described. The attention was drawn to the necessity of finding agreement between the real separation criterion and the method in which the particle population is characterized for the purpose of analysis and modelling of the flow processes. The characterisation method for particle populations has been proposed. The method is a combination of two basic particle population characterisation methods i.e. size analysis and sink and float test. The principle of the method is the calculation of the settling velocity of narrow particle size and density fractions in the separation medium and, next, arranging the separated size and density fractions according to the settling velocity. The above methodology was applied in characterizing two various coal slurries. The slurries were next separated in two various realizations of flow processes. One type of slurry was separated in the classifying and thickening HC-100 type hydrocyclone and the other one in HWO-300 type water-only cyclone. Prognoses were prepared, concerning the theoretical results of ideal separation (excluding the randomness of industrial processes) of both slurries according to the hitherto applied methods and according to the method, based on the distribution of settling velocity of particles. The results of prognoses were compared with the results of real separations. The method for characterising the particle population provides more complete and more accurate prognoses of the results of separation in hydrocyclones. One of the method's advantages is the conformity of the particle population characterisation method with real separation criterion in flow processes. The paper is concluded by a discussion of faults and uncertainties of the method for characterizing particle populations. (C) 2012 Elsevier B.V. All rights reserved.