A mass spectrometer experiment for the analysis of minor constituents in the stratosphere has been flown successfully four times from Palestine, TX on board a balloon gondola. The carbon dioxide mixing ratio, which shows unexpectedly large variations in the stratosphere, reached 400 ppm in one particular night flight. This is about 20% higher than the ground value. Evidence is presented that the experiment performed well during each of the balloon flights. The isotopic ratio 12C/13C was measured and found in good agreement with previous air analyses showing a depletion of 13C.