A model based on two-phase volume-averaged equations of motion is proposed to examine the gravity dependence of the bubble-to-pulse transition in gas-liquid cocurrent down-flow through packed beds. As input, the model uses experimental correlations for the frictional pressure drop under both normal gravity conditions and in the limit of vanishing gravity, as well as correlations for the liquid-gas interfacial area per unit volume of bed in normal gravity. In accordance with experimental observations, the model shows that, for a given liquid flow, the transition to the pulse regime occurs at lower gas-flow rates as the gravity level or the Bond number is decreased. Predicted transition boundaries agree reasonably well with observations under both reduced and normal gravity. The model also predicts a decrease in frictional pressure drop and an increase in total liquid holdup with decreasing gravity levels. (c) 2013 American Institute of Chemical Engineers AIChE J 60: 778-793, 2014