Abstract: A method for measuring and controlling the oxygen concentration in silicon melts which are contained in a silica glass crucible or which can be touched by a silica glass surface can especially be used for achieving a defined, uniform axial and radial oxygen concentration in a growing silicon crystal. The measurement of the oxygen concentration is realized by an electrochemical solid ionic sensor dipped into the melt; the voltage is measured between the sensor and the growing silicon crystal. The oxygen concentration in the melt is controlled by applying a voltage between crystal and silica glass crucible. The apparatus for putting the method into practice includes an electrochemical solid ionic sensor formed of a silica glass tube which encloses a metal/metal oxide mixture, contacted by a metal wire.