In the petrochemical industry area, many processes are carried out using fixed bed reactors with cocurrent upward or downward gas/liquid flows. One of the main problems to overcome during the scale-up of such reactors is to optimize the gas/liquid flow distribution across the reactor catalytic bed section. To accurately characterize the gas/liquid flow distribution inside a real catalytic bed, a new computed gamma-ray tomographic system has been developed to investigate two-phase flow through cold mock-ups of large scale (60 cm in diameter). In the case of developing a new reactor technology for processes working with fixed bed in the trickling flow regime, this type of measurement is quite important. This system, unique for such a large size catalyst bed, includes a series of 32 BGO detectors and a Cs-137 source. The performances of this experimental system have been fully evaluated through a validation including reconstruction of physical phantoms in static conditions. Several tests have been performed to measure the gas/liquid flow distribution in the catalyst bed for different inlet distributor configurations. It is shown, in the experiments performed for the trickle flow regime, that the instrumental technique has a very good sensitivity to liquid retention measurement. When the liquid retention measurements are compared with a hydrodynamic model based on Kozeny-Carman formalism a quantitative agreement is observed.