One possible source of error in rheological characterization of concentrated suspensions using nonhomogeneous flows is the migration of particles from high to low shear rate regions to give rise to concentration gradients in the specimen. Numerical data are presented and simple experimental procedures rue suggested for the assessment of the particle-migration effects in pressure-driven viscometers including capillary and rectangular slit viscometers. (C) 2000 The Society of Rheology. [S0148-6055(00)00401-6].