The partial dissolution of crystalline material during filter cake washing can affect greatly on the cake permeability in separation and purification processes. Besides decreasing the cake thickness, the dissolving may cause changes in the cake structure as it may alter the particle size distribution based on the fact that different sized particles have different dissolution rates. This is especially the case if the cake consists of particles having a wide particle size distribution. The present work introduces a model based on mass transfer to predict the dissolution rates of crystalline material having different solubility in various washing liquors. The filtration system studied experimentally was ground sucrose filtered from ethanol solution. In addition, inline monitoring of the filtration and cake washing with a Raman spectrometer immersion probe was carried out. The studied impurity was cyclohexane. The measuring method allowed measuring of the ternary compound system: the impurity, the concentrations of the mother liquor and the solid in the filter cake.