Ultrafast magnetic resonance imaging (MRI) is used to provide two-dimensional (2-D) images of gas-liquid distribution within trickle-bed reactors with data acquisition times of 20 and 40 ms. Gas-water, cocurrent downflow through a fixed bed of cylindrical porous pellets of length and dia. 3 mm, packed within a 43 mm internal dia. column, was studied in both the trickle- and pulsing-flow regimes. Superficial gas velocities in the range 50-345 mm s(-1) (0.06-0.42 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. MRI is used to investigate the stability of the gas-liquid distribution in the trickle- and pulsing-flow regimes. At the onset of the transition to pulsing flow, local pulsing, at the length-scale of the size of the packing elements is observed within the bed. Increasing liquid velocity causes an increase in the number of these local pulses until a velocity is reached at which the system transforms to the rapidly-changing gas-liquid distribution typical of pulsing flow. (C) 2005 American Institute of Chemical Engineers.