A 26 mm N.B. horizontal pipe loop has been used to study the transport velocity of aqueous dispersions of colloidal silica. At low electrolyte concentrations the silica spheres form a stable dispersion, while at high electrolyte concentration particle aggregates are readily formed. Two measurement techniques have been used to study the critical transport velocity; (i) flow visualisation, and (ii) ultrasonic Doppler velocity profiling. Both measurement techniques show agreement in identifying the critical transport velocity. The study has shown that the critical transport velocity can be reduced when the suspension is transformed from a dispersed (d(50) = 0.8 mu m) to an aggregated (d(50) = 4.6 mu m) state. Such behaviour is related to the introduction of inter-floc flow through aggregates, enhancing the turbulence intensity of the fluid. (C) 2011 Elsevier Ltd. All rights reserved.