Abstract: The present invention relates to a gassing unit for bubble-free introduction of a process gas into a liquid located in a reactor, wherein the gassing unit comprises at least: a first gas receiving chamber and, spaced therefrom, a second gas receiving chamber for receiving a process gas, the two gas receiving chambers being connected to one another via at least two two-dimensional, gas-conducting diffusion membranes comprising hollow fibers spaced apart from one another and at least partially fixed to one another; a receptacle for a gas supply on at least one of the gas receiving chambers; a receptacle for a shaft on at least one of the gas receiving chambers; wherein the gassing unit for gassing the liquid in the reactor can be supplied with process gas via the gas supply receptacle, can be set into a rotational movement via the receptacle for the shaft and can form a convection flow within the reactor via the rotational movement of the gassing unit in the liquid.