In this paper, we propose a new experimental technique, based on X-ray tomography, allowing the analysis of the gas-liquid-solid phase distribution in trickling flow reactors. Tomography refers to the cross-sectional imaging of an object from data collected by illuminating this object from many directions. The experimental set-up allows the scanning of transverse sections of rather large columns : up to 0.8 m diameter and 2 m height can be irradiated. Experiments were carried out both without and with liquid flow. In the absence of liquid, the reconstructed images give a view of the non-uniformity of the solid phase distribution. The images obtained are very complex because they result from the superposition of several distributions with different Spatial scales. We show the existence of a first spatial scale which allows us to separate properties belonging to the individual packing elements from those belonging to the bed scale. When liquid is flowing through the packed bed X-ray scans give a clear picture of the distribution of the liquid phase saturation, despite the very low liquid film thickness. Average values of this liquid phase saturation are determined and confronted with a partial wetting model.