Abstract.A portable Fourier transform spectrometer (FTS), model EM27/SUN, was deployed onboard the research vessel Polarstern to measure the column-average dry air mole fractions of carbon dioxide (XCO 2 ) and methane (XCH 4 ) by means of direct sunlight absorption spectrometry. We report on technical developments as well as data calibration and reduction measures required to achieve the targeted accuracy of fractions of a percent in retrieved XCO 2 and XCH 4 while operating the instrument under field conditions onboard the moving platform during a 6-week cruise on the