In many crystallization processes and other solid-liquid reactors, solids are present in high concentrations. Often these reactors are operated in the semi-batch mode and the purity, productivity and selectivity of the reaction (and in the case of precipitation and crystallization, the size distribution and morphology too) depends on the relative rates of mixing (or homogenization) compared to chemical reaction. It has usually been assumed that, provided the solids are all fully suspended, the rate of mixing is similar to that found in single-phase systems. Recent extensive work carried out by the above team at The University of Birmingham with support from DuPont has analysed the processes of homogenization and particle suspension. It has shown that, for typical industrial conditions, there is a range of operating conditions in which the solid particles at high concentration are fully suspended but have a clear liquid layer above them in which the local specific energy dissipation rate appears to be very low. Under these conditions, the mixing time may be two or more orders of magnitude longer than in the single phase case. This paper describes the phenomenon, analyses its importance, develops a physical model and proposes ways of overcoming the problem.