Thyratrons are limited in lifetime and maximum repetition rate. For some applications it is necessary to replace the thyratron by a semiconductor switch. Therefore it is necessary to compress the pulses by nonlinear inductances. In a magnetic pulse compression network three main losses exist. In principle the copper conducting losses can be infinitely reduced. The losses in the capacitors are very small and can principally also be reduced to an infinite degree. But the core losses are different. Due to the fact that the material is magnetized from one saturation to the other, the losses depend only on the material and the magnetization speed. The losses in these cores essentially determine the max. attainable pulse frequency. A procedure is presented on how these losses can be easily determined. Different materials are presented and also their behavior under different conditions.