This article presents an approach to the modeling of CaCO3 fouling and cleaning in a microscale cross-flow heat exchanger. The fouling progress was detected by thermal, fluid dynamical, and optical measures. In general, the observed fouling phenomena at microscale were comparable to those at macroscale. The detected thermal fouling resistance was between 10(-6) and 10(-3) m(-2) K-1. As expected, crystallization fouling strongly depends on the surface temperature. In addition, the surface coverage can be a useful measure for both processes-fouling and cleaning. An extended approach to derive a reasonable fouling allowance was formulated through a fractional fouling resistance.