[en] During the reprocessing of spent nuclear fuel by liquid-liquid-extraction the formation of crud and in consequence an interruption of work may occur in the system 30% TBP/dodecane/nitric acid. Chemical and physical backgrounds were studied in simplified systems. The most important results obtained are: It has been concluded from this study that the crud is caused by Zirconium-Phosphate (Zr₃(PO₄)₄) which is in contradiction to the prevailing findings that the emulsive agency of Zirconium-Dibutylphosphate (Zr-DBP) was the responsible for the crud formation; The crud formation is a result of the stabilization of the mixed phase by a dispersed solid matter; A tenside working of Dibutylphosphate and Monobutylphosphate was not proved and in addition to result ''1'' it was found that in systems with low acidity (<= 0,1 Mol/l HNO₃) the precipitation of thorium and iron with DBP and MBP are found to cause the crud formation. The physical properties of the precipitates e.g. surface area, density, the rate of dispersion and wetting characteristics are being investigated. (orig.)