The flow of dilute aqueous suspensions of micron-size carbon black particles in diffusers was studied by visualization and by measurements of pressure drop. For particle concentrations from 0.1 to 1.0% by weight and for conical diffusers with half angles of 5 and 11 degrees, the measurements show that pressure recovery was less than that for plain water. The reductions were of order 10% and increased with particle concentration. To understand this effect, the flow was visualized in a two-dimensional diffuser, using computed sonography because of liquid opacity. The sonographic images show that the flow separated earlier than it did with water, which explains the reduced pressure recovery. At the same time, it is not understood how such minute particles at such small concentrations affect flow separation. (C) 2000 Elsevier Science Ltd. All rights reserved.