For heterogeneously catalyzed multiphase reactions the formation of bubbles may have an influence on mass and heat transfer as well as on the effective reaction rate. This first of two contributions deals with the Ni-catalyzed decomposition of H2O2, which was used as a model system for an (almost) isothermal reaction with a gaseous product. (In part lithe strongly exothermic hydrogenation of hexene will be analyzed, where gas/vapor bubbles may be generated by overheating of the catalyst.) The discharge of O-2 bubbles formed by decomposition of H2O2 enhances the external mass and heat transfer up to one order of magnitude. This is in analogy to the well-known phenomena during nucleate boiling. The experiments and theoretical considerations also show that the internal mass transfer depends on the intensity of the reaction and thus on the H2O2 concentration, which is in contradiction to the classical Thiele approach. This discrepancy could be explained by a modified model that takes the formation of bubbles into account.