A peculiar phenomenon is observed when a binary-solid bed, in which the particle species possessing the larger minimum fluidization velocity is placed initially on top of the other, is brought from the fixed to the fluidised state. Contrary to the well known behaviour of mono-component beds, a single minimum fluidization velocity is not observed in this case and, more importantly, the fluid pressure drop can turn out to be considerably greater than the total effective weight of the particles. Particle-wall interactions are shown to be responsible for this latter effect. The problem is approached by considering differential horizontal slices of the bed with the simplification suggested by Janssen in 1895 that the ratio of vertical to horizontal solid pressure in a slice is constant-the Janssen constant K. Predictions of the model, which contains no adjustable parameters, are shown to be in excellent agreement with experimental measurements. (C) 2010 Elsevier Ltd. All rights reserved.