Magnetic resonance imaging (MRI) has recently been used to image local pulsing events, which are associated with the transition from trickle to pulsing flow in fixed-bed reactors. This paper reports ultrafast MRI measurements of the liquid distribution in two and three dimensions within a 43-mm-internal-diameter column packed with cylindrical porous pellets of length and diameter 3 mm, operating under conditions of cocurrent, air-water downflow. Superficial gas velocities in the range 25-300 mm s(-1) (0.031-0.367 kg m(-2) s(-1)) and superficial liquid velocities in the range 0.4-13.3 mm s(-1) (0.4-13.3 kg m(-2) s(-1)) were used. The data acquisition times used were 20 and 280 ms, giving two- and three-dimensional spatial resolutions of 1.4 mm x 2.8 mm and 3.75 mm x 3.75 mm x 1.87 mm, respectively. The 3D data enable us to quantify the spatial extent and number of local pulsing events as a function of the bed operating conditions. By following the number of isolated local pulsing events as a function of increasing liquid velocity for a constant gas velocity, it is possible to make an objective characterization of the transition point; this is defined as the liquid velocity at which the number of isolated pulsing events reaches a maximum before decreasing as a result of individual pulses merging together. Well-defined, local temporal correlations in the liquid content are also observed in the trickle-flow regime, suggesting that the local pulses originate from instabilities in liquid films present on the surface of the catalyst pellets. These data provide strong evidence in support of earlier theoretical studies proposing that the trickle-to-pulse transition originates from film instabilities on the surface of catalyst pellets comprising the bed and, hence, that the transition itself is best modeled by considering the pore-scale characteristics of the trickle-bed reactor.