Abstract. The Thermo Scientific 450 Hydrogen Sulfide–Sulfur Dioxide Analyzer measures both hydrogen sulfide (H₂S) and sulfur dioxide (SO₂). Sulfur dioxide is measured by pulsed fluorescence, while H₂S is converted to SO₂ with a molybdenum catalyst prior to detection. The 450 is widely used to measure ambient concentrations, e.g., for emissions monitoring and pollution control. An air stream with a constant H₂S concentration was generated and the output of the analyzer recorded as a function of relative humidity (RH). The analyzer underreported H₂S as soon as the relative humidity was increased. The fraction of undetected H₂S increased from 8.3 at 5.3 % RH (294 K) to over 34 % at RH  >  80 %. Hydrogen sulfide mole fractions of 573, 1142, and 5145 ppb were tested. The findings indicate that previous results obtained with instruments using similar catalysts should be re-evaluated to correct for interference from water vapor. It is suspected that water decreases the efficiency of the converter unit and thereby reduces the measured H₂S concentration.