Magnetic resonance imaging was used to study the impact of Ca2+ on the fouling behavior of alginate in dead-end and cross-flow filtrations. In situ experiments were performed to measure alginate layer formation and flow behavior during filtration. It was found that magnetic resonance imaging allows for distinguishing different structures of the formed alginate layers. The structure of the alginate layer without the addition of Ca2+ is a fluid and unordered concentration polarization layer, whereas a dense and stable gel layer is formed on the membrane's surface in case of the filtration of alginate with Ca2+. This was in good agreement with data of filtration experiments. To overcome the lack of contrast between alginate and surrounding water, the addition of specific contrast agents was necessary in the case of dead-end filtration. Using tagging methods, the flow profiles during dead-end filtrations were investigated, and it was shown that the addition of Ca2+ not only influences the fouling layer structure, but also the flow behavior. Furthermore, cross-flow experiments were performed. Here, the outflow effect was successfully used as contrast mechanism.