In modelling gas/liquid reactors the gas hold-up and the internal circulation flow are the most important parameters of reaction engineering. These parameters can vary a lot according to the design of the reactor,In this paper a mechanical power balance for the most important types of aeration is presented, which can be solved directly only for special cases. For this reason new physically based characteristic numbers are introduced, which allow an easy empirical description. All types of reactors discussed in this paper are arranged in a graphical presentation, where stable operating conditions can be shown.