The intention of this work is the basic investigations of the fouling behavior in micro heat exchangers. Therefore fouling experiments with calcium carbonate (CaCO3) in an experimental micro heat exchanger were carried out and observed with a digital microscope. The investigations included local temperature measurements confirmed by computational fluid dynamics simulation as well as optical visualization of the fouling process inside the microchannels. The detected fouling resistances R-f were in the range of 10(-5)-10(-4) m(2) K W-1. Cleaning in place was possible and also optically observed.