Hydrogen is an important source material in the chemical industry and plays an important role in peripheral and regenerative energy supply like e.g. fuel cells. In particular for hydrogen electrolysis the oxygen concentration is a crucial parameter in respect of the quality of the produced hydrogen and safety aspects. Here we present a spectroscopic sensor, which is appropriate for online-monitoring of oxygen at high pressures. It shows no cross-sensitivities to water and hydrogen, and its optical power is well below the limit of explosion. (C) 2009 Published by Elsevier B.V.