Bubble columns are an important device for multi-phase reactions or reactive separation processes in chemical engineering. For the design of bubble columns one can use empirical correlations which consider values like superficial gas velocity or gas holdup. These integral parameters can be measured quite well. But due to complex interactions at the bubble surface these integral parameters are insufficient to describe the behavior of bubble columns. This paper will present some measurement methods which can be used to determine the phase fractions of gas and liquid over the entire cross section of a bubble column in pilot-plant scale. In particular, the wire mesh sensor used in own experiments is discussed.