Abstract From the data of 25 deep-sea cores it is possible to show that the U/Th ratio of sediments is nearly constant at a low CaCO 3-content. This result and reflections about some other points have led to the assumption that U and Th are being transported into the sediment through the intermediary of minerals. During the transport U (preferably 234U), Ra, and some Th are leached by ocean water. This model provides for a possibility to interpret the activity ratio 234U/ 238U< 1 in deep-sea sediments with low carbonate content as well as the activity ratio 228Th/ 232Th∼ 15 in ocean water. Likewise a part of the 226Ra excess (compared with the Io in solution) may be caused by that leaching.