Particle synthesis in liquid phase usually requires a subsequent washing step in order to remove undesirable impurities such as unreacted educts, byproducts, solvents and salts. In particular, the washing of mesoporous particles is a challenging task due to large specific surface areas and void volumes. The filter cake washing of mesoporous SiO2 particles contaminated either with an adsorbing fluorescent dye or sodium sulfate is presented. The effects of wash liquid mass flow rate, mean particle size, and mean pore size on the wash efficiency were investigated. Especially the interaction between impurities and the solid surface, and thereby the initial impurity distribution within the filter cake, led to distinctive washing behaviors.