The formation of local instabilities, identified as isolated pulsing events, in the gas-liquid distribution within a trickle-bed reactor is shown to be central to the mechanism of the hydrodynamic transition from trickle-to-pulsing flow. The evolution of these instabilities as the bed moves through the transition is imaged in 3-D using ultrafast magnetic resonance imaging (MRI). The reactor was of internal dia. 43 mm and packed with cylindrical packing elements of length and diameter 3 mm, operating under conditions of air-water, cocurrent downflow. Superficial gas and liquid velocities in the range 25-300 and 0.9-13.0 mm s(-1), respectively, were used. For a given gas velocity, the transition point is defined to be the liquid velocity at which the maximum number of isolated pulses occurs. The transition point determined using this approach is critically compared with the predictions of existing correlations and theoretical models. (c) 2005 American Institute of Chemical Engineers.