The TRANSDRIFT IV expedition was part of the joint Russian-German cooperation and the Laptev Sea System project. The Arctic and Antarctic Research Institute (AARI) in St. Petersburg and the GEOMAR Research Center for Marine Geosciences were jointly responsible for the organization and coordination of the TRANSDRIFT IV expedition, which was funded by the Russian Ministry for Research and Technology and the German Ministry for Research and Technology.The TRANSDRIFT IV expedition was carried out from the fast ice, pack ice and in the Lena Delta near the "Lena Nordenskøld" station in the Laptev Sea from May 18 to June 12, 1996. A multi-disciplinary working program investigated the causes and effects below the annual fast ice, the pack ice and in the polynya during late winter and river break-up. Unlike our previous expeditions to the Laptev Sea, which focused on oceanographical, hydrochemical, ecological and sedimentological processes during the brief ice-fee period in summer and during freeze-up in fall (1995), this expedition studied these processes during the extreme physical conditions in late winter just before the onset of fast-ice breakup (May 18-22) and also during different stages of river break-up (June 3-6 and June 10-11). This was the first study of its kind under these conditions.The working program was carried out in bilateral cooperation. CTD data were retrieved at a total number of 44 positions in the Laptev Sea. Hydrochemical sampling with oxygen, silicate and phosphate analysis was carried out at all of these stations.