[en] The periodically pulsed IBR-2 reactor of the Joint Institute for Nuclear Research in Dubna (Russian Federation) uses liquid sodium as the coolant for the fuel elements. The installation of sodium circuits in the reactor system poses numerous tasks in regard of control and safety. Electrochemical cells with solid electrolytes were installed for the measurement of the oxygen activity in the sodium of the primary circuit. The carbon contained in the sodium was determined by sampling using the vacuum distallation method. Metallic impurities were ascertained by atomic emission spectometry while radiochemical impurities were analysed on whole samples or from the distillation residues. The results of the investigations are presented and discussed. (orig.)