Apparatus for the determination of fill levels of liquid layers (4, 6; 54, 56; 84, 86, 88) arranged on top of each other in a container (2; 52; 70; 82), one liquid layer (4; 54; 56; 86) being electrically conductive, comprising - a magnetostrictive position measuring system (30, 32) comprising a measuring wire and first operating electronics and - at least one buoyancy body (34; 64; 94, 95) having a magnet adjusted to the magnetostrictive position measuring system (30, 32) and being adapted to float at a boundary (7; 57; 85, 89) between two layers of different density, characterized - by a potentiometric measuring system comprising an electrode (10; 60; 90) having two ends (28, 29; 62, 63; 72, 73; 92, 93), which forms a resistor and is adapted to immerse in the liquid, a counter electrode and second operating electronics, - and in that - either the electrode (10; 60; 90) is designed as a guiding tube for the at least one buoyancy body (34; 64; 94, 95), the measuring wire of the magnetostrictive position measuring system (30, 32) being guided through the interior space of the electrode (10; 60; 90), - or the counter electrode is designed as a guiding tube for the at least one buoyancy body, the wall of the counter electrode being perforated to provide access of liquid to the electrode arranged in the interior space of the counter electrode, preferably concentrically thereto, the electrode having a tubular shape and containing, in its interior space, the measuring wire of the magnetostrictive position measuring system. - a magnetostrictive position measuring system (30, 32) comprising a measuring wire and first operating electronics and - at least one buoyancy body (34; 64; 94, 95) having a magnet adjusted to the magnetostrictive position measuring system (30, 32) and being adapted to float at a boundary (7; 57; 85, 89) between two layers of different density, - by a potentiometric measuring system comprising an electrode (10; 60; 90) having two ends (28, 29; 62, 63; 72, 73; 92, 93), which forms a resistor and is adapted to immerse in the liquid, a counter electrode and second operating electronics, - and in that - either the electrode (10; 60; 90) is designed as a guiding tube for the at least one buoyancy body (34; 64; 94, 95), the measuring wire of the magnetostrictive position measuring system (30, 32) being guided through the interior space of the electrode (10; 60; 90), - or the counter electrode is designed as a guiding tube for the at least one buoyancy body, the wall of the counter electrode being perforated to provide access of liquid to the electrode arranged in the interior space of the counter electrode, preferably concentrically thereto, the electrode having a tubular shape and containing, in its interior space, the measuring wire of the magnetostrictive position measuring system. - either the electrode (10; 60; 90) is designed as a guiding tube for the at least one buoyancy body (34; 64; 94, 95), the measuring wire of the magnetostrictive position measuring system (30, 32) being guided through the interior space of the electrode (10; 60; 90), - or the counter electrode is designed as a guiding tube for the at least one buoyancy body, the wall of the counter electrode being perforated to provide access of liquid to the electrode arranged in the interior space of the counter electrode, preferably concentrically thereto, the electrode having a tubular shape and containing, in its interior space, the measuring wire of the magnetostrictive position measuring system.