The formation of a filter cake during mechanical dewatering (filtration and expression) of sludge is studied using one-dimensional nuclear magnetic resonance (NMR) imaging of porosity profiles. The experiments clearly showed the formation of a porosity gradient during filtration and the disappearing of this gradient during expression, forming a cake with a uniform porosity. This technique makes it possible to investigate the material behavior of the porous medium, which is needed before any model calculations can be performed. The results indicate that NMR imaging is useful in nondestructive monitoring of the filtration and expression process of sludge.