We investigated the local hydrodynamics and phase distribution of complex polydisperse suspensions 'just-suspended' by a down-pumping pitched blade impeller in a mechanically agitated vessel. The solid-liquid mixtures consisted of five size fractions of coarse glass particles in water, and the total concentration was varied within the range 2.4-23.6 vol% (5-40 wt%). The whole flow field and spatial distribution of the liquid phase and of each particle size fraction were resolved using the Lagrangian technique of positron emission particle tracking (PEPT). For the first time, it has been possible to conduct such a detailed 'pointwise' examination within an opaque polydisperse suspension of this type and complexity. For each component of the two-phase flow, results are presented in the form of maps of local velocity, solid concentration and time-averaged slip velocity, as well as plots of spatial suspension uniformity index. The accuracy of measurements is verified through six-component mass balance and mass continuity calculations. (C) 2015 Elsevier Ltd. All rights reserved.