Abstract: Plasmonic catalysis, which is driven by the localized surface plasmon resonance of metal nanoparticles, has become an emerging field in heterogeneous catalysis. The microscopic mechanism of this kind of reaction, however, remains controversial partly because of the inaccuracy of temperature measurement and the ambiguity of reagent adsorption state. In order to investigate the kinetics of plasmonic catalysis, an online mass spectrometer-based apparatus has been built in our laboratory, with emphases on dealing with temperature measurement and adsorption state identification issues. Given the temperature inhomogeneity in the catalyst bed, three thermocouples are installed compared with the conventional design with only one. Such a multiple-point temperature measuring technique enables the quantitative calculation of equivalent temperature and thermal reaction contribution of the catalysts. Temperature-programmed desorption is incorporated into the apparatus, which helps to identify the adsorption state of reagents. The capabilities of the improved apparatus have been demonstrated by studying the kinetics of a model plasmon-induced catalytic reaction, i.e., H₂+D₂→HD over Au/TiO₂. Dissociative adsorption of molecular hydrogen at Au/TiO₂ interface and non-thermal contribution to HD production have been confirmed.