The potential of magnetic resonance imaging (MRI) measurements to investigate gas and liquid distributions within a fixed-bed reactor operating in co-current downflow in the trickle-flow regime is demonstrated. Liquid holdup and wetting efficiency are studied for a range of gas (66-356 mm/s) and liquid (0.5-5.8 mm/s) superficial velocities. Two-dimensional and three-dimensional (3-D) magnetic resonance images have been acquired for flow within a packing of 5 mm diameter glass ballotini contained in a cylindrical column of internal diameter 40 mm. The images are of sufficiently high resolution to be able to characterise liquid rivulets within the bed and to detect the presence of thin water films on the surfaces of the packing elements. As expected, liquid holdup and wetting efficiency increase with increase in liquid flow rate, at a fixed gas flow rate. A transition in the holdup and wetting characteristics is observed at a liquid superficial velocity of 1.5-2 mm/s. The number of liquid rivulets increased rapidly as the liquid superficial velocity was increased from 0.5 to 2.0 mm/s and then reached a plateau; the increase in liquid flow as the liquid flow rate was further increased was taken up in the increasing size of existing rivulets. Differences in the nature of the liquid distribution within the bed are also reported for different conditions of prewetting of the packing. Gas flow rate affected the onset of flow instabilities but not the liquid distribution. 3-D MRI data enabled the visualisation of the extent of liquid filling of individual 'pores' within the inter-particle space as a function of liquid flow rate, and how this depended on the 'pore' size. For each packing element, the fraction of the surface area that was wetted was found to correlate with the number of contact points that packing element made to other packing elements within the bed.