A home-made analyzer for dissolved gasses in water, based on membrane inlet mass spectrometry (MIMS), was successfully applied for the first time as an in-line method for detection of gases dissolved in an aqueous stream, at the exit of a catalytic reactor in a transient experiment. The technique was applied for measuring the metal dispersion of three different catalysts (EuroPt-1, Pt/SiO2 and Pt/CNF) in liquid phase via H2O2 titration. The titration stoicheometry obtained (i.e. ratio of H2 and O2 consumption) is in agreement with the stoicheometry obtained in gas phase titration experiments in literature. The platinum dispersions obtained in liquid phase are in excellent agreement with dispersions based on hydrogen chemisorption in gas phase for all three catalysts. Clearly, water does not influence the accessibility of the Pt atoms, disagreeing with recent suggestions in literature. In addition, the detector is universal and allows calibration of H2, O2, N2 and Ar dissolved in water with insignificant cross contamination. Despite the relatively slow dynamic response of the detector (between 4 min for hydrogen up to more than 40 min for oxygen), it was proven possible to perform quantitative experiments in pulse mode, with a detection limit in the order of 0.5 m2 Pt. The capability of this method is further demonstrated by the surprising observation that dissolved hydrogen physisorbs significantly on a reduced Pt catalyst whereas dissolved oxygen does not interact with an oxidized catalyst.