The formation of caverns around the impeller is a characteristic of the mixing of non-Newtonian fluids which exhibit an apparent yield stress. Viscous non-Newtonian fluids which are not viscoplastic show pseudo-caverns instead, especially in the laminar and transitional flow regimes. The formation of caverns in a yield-stress fluid of the Herschel-Bulkley type, and pseudo-caverns in a shear-thinning power-law fluid, was studied in the laminar and transitional flow regimes using a CFD model. A PLIF technique was used to experimentally visualise and hence measure caverns and the rate of fluid mixing in the yield stress fluid. The MFR configuration was employed in conjunction with the laminar model and the apparent viscosity function of the fluid to solve the flow field using the CFX software. Mixing within caverns as indicated by PLIF was very slow. CFD predictions of cavern size agreed very well with experimental measurements at low Reynolds numbers. A number of theoretical cavern/pseudo-cavern models were compared to CFD, and experiment in the case of caverns, with the toroidal model giving the better agreement in terms of shape and size for both types of fluid.