Fouling processes present significant challenges in many industries; understanding these processes is crucial for accurate prediction and planning effective mitigation protocols. Composite fouling is prevalent in many industrial applications, however, studies of these systems are scarce. We present, for the first time, new insights on the interaction between two common crystallization foulants: BaSO4 and CaCO3. We conducted a series of crystal growth experiments in a plug flow reactor. We used high resolution X-ray micro-computed tomography to visualize the crystal formation. In the first set of experiments, crystals (either BaSO4 or CaCO3) were grown on pristine steel surfaces; these pre-fouled surfaces were then used as substrates for the composite fouling studies. The chemical compositions of the substrates were different from that of the foulant, i.e., either BaSO4 on CaCO3 or CaCO3 on BaSO4. Growth rates were obtained through gravimetric analysis. We characterized surface properties, such as the crystal size distribution, surface coverage, and texture parameters through advanced image analysis. Our results reveal that deposition behaviour depends on complex substrate-foulant interactions. Whereas the net deposition of CaCO3 on a BaSO4 substrate increased with time, significant detachment processes were observed for the reverse case. The increased detachment of BaSO4 on a CaCO3 substrate can be attributed to either the weak interaction within the CaCO3 crystal structure or the CaCO3/steel interface. We also show how substrate properties govern crystal cluster sizes and distribution. The results from this work provide a basis for the development of more comprehensive prediction models. Display omitted •Composite fouling systems were investigated•BaSO4 substrates nearly doubled the growth rate of CaCO3.•CaCO3 substrates promote BaSO4 detachment.•BaSO4 preferentially grew on top of CaCO3 resulting in smaller clusters.