The hydrodynamics of trickle bed reactors operating near the trickle-bubbly flow regime transition have not been fully characterized in the literature. In this work, an air-water system is used to investigate the effects of particle size, particle shape, void fraction, and surface tension on the trickle-bubbly flow regime transition in trickle bed reactors. The flow regime transition is detected based on standard deviation of pressure drop with confirmation by visual observation. For all cases, the liquid superficial velocity required for the trickle-bubbly transition was found to be relatively independent of the gas superficial velocity (nu(G) = 4-40 mm/s). Literature models, defined for the trickle-pulse transition, are unable to predict this trend when extrapolated to low gas superficial velocities (nu(G) <= 40 mm/s). To address this gap, a correlation is proposed based on the experimental data gathered in this work. (C) 2015 Elsevier B.V. All rights reserved.