In the sedimentation of suspensions, the particles modify their settling velocity in accordance with the solids concentration, Kynch's theory assumes that in each layer, the settling velocity only depends on the solids concentration, and neglects the acceleration and deceleration processes when the particles descend with a velocity distinct from that corresponding to the solids concentration at this layer. The Kynch theory also assumes that there are so many small particles that the solid mass can be considered homogeneously distributed in the suspension, so the particulate character of the suspension is ignored. Kynch's theory also neglects the diffusion effects. An analysis of the particulate character of the suspension, the inertial effects and the diffusion effects are discussed in this paper, deducing a critical value for considering the particulate character negligible and a critical size for considering the diffusion effect also negligible. Finally, several data of different flocculated suspensions are satisfactorily analysed in view of the conclusions obtained previously.