Nuclear magnetic resonance (NMR) micro-imaging has been used to investigate concentration polarization phenomena in membrane filtration of colloidal silica suspensions using a single tubular microfiltration membrane, with the feedstock fed to the inner lumen of the membrane and the filtrate removed from the (outer) shell side. H-1 NMR images, in which the signal intensity is weighted by the longitudinal relaxation time (T-1) of the solvent (water) protons, clearly exhibit details of the formation and dissipation of the silica particle concentration polarization layers at the surface of the membrane in response to changes in trans-membrane pressure difference and feedstock crossflow rate. The images were used to map the spatial distribution of the silica polarization layer as a function of time, distance from the filter inlet, and applied trans-membrane pressure difference. In each case the polarization layer was observed to be highly asymmetric, being much thicker at the bottom of the module than at the top.The performance of the filter was compared for different orientations of the filter module. The permeate flux rate was shown to be highly dependent on the orientation of the filter axis with respect to the vertical, This is consistent with the fact that the observed asymmetry in the layer is caused by flow of the polarization layer over the surface of the membrane due to gravitational effects.Phase sensitive NMR flow imaging was used to map the ID distribution of the feedstock crossflow on the lumen side of the membrane as well as measuring the axial flow profile within the concentration polarization layer itself. The axial component of flow of the polarization layers is driven, not by gravity, but by the shear induced by the feedstock crossflow. The flow profile of the polarization layer presented in this paper therefore provides direct experimental evidence for fluidity and motion of concentration polarization layers, an assumption which has been invoked for the development of some theoretical models but which has not previously been confirmed experimentally.