The development of flow instabilities during the capillary flow of two concentrated suspensions filled with 76.5 and 65.6019 by volume solids was investigated. The flow instabilities manifested themselves by the development of concentration gradients as a result of the filtering of the binder, superimposed on the bulk motion of the suspension. The effects of apparent shear mte, capillary diameter and the surface roughness of the particles were investigated. The use of the comparison of the filtration rate with the bulk velocity of the suspension during flow is shown to be promising for the prediction of the apparent shear rate at which filtration-based flow instabilities occur.