  We report on an initial analysis of Herschel/HIFI observations of hydrogen
chloride (HCl), hydrogen peroxide (H_2O_2), and molecular oxygen (O_2) in the
Martian atmosphere performed on 13 and 16 April 2010 (L_s ~ 77{\deg}). We
derived a constant volume mixing ratio of 1400 +/- 120 ppm for O_2 and
determined upper limits of 200 ppt for HCl and 2 ppb for H_2O_2. Radiative
transfer model calculations indicate that the vertical profile of O_2 may not
be constant. Photochemical models determine the lowest values of H_2O_2 to be
around L_s ~ 75{\deg} but overestimate the volume mixing ratio compared to our
measurements.
