As part of the guaranteed time key programme "Water and related chemistry in the Solar System" (KP-GT HssO, [1]) Mars has been observed during two periods in April and June 2010 with the HIFI instrument [2] of the Herschel Space Observatory [3]. We observed several rotational transitions of H2O and its isotopes H17 2 O, H18 2 O, and HDO at the very high frequency resolution of 1MHz and with a instantaneous frequency coverage of 4 GHz. These observations allow not only to obtain information about the vertical distribution of water vapour, but allow also to determine the isotopic ratios 17O/16O, 18O/16O, and D/H. To constrain the temperature structure of the Martian atmosphere between ground and about 80 km altitude additional observations of several rotational transitions of CO and its isotopes C17O, C18O, and 13CO have been performed within a few days of the water vapour observation. First results of the CO observations have been already published by Hartogh et al. [4]. Using our standard line-by-line radiation transfer model we analyze the complete set of data to derive a new set of isotopic ratios. The additional observations of CO transitions allow to investigate the consistency of the oxygen istotopic ratios as obtained from two different constituents of the atmosphere. Finally we will carefully explore the data for indications of possible deviations from a uniform mixing of the isotopologues< Leer menos