Bubble size distributions in a two-dimensional packed bed are investigated as a function of axial direction by using image processing techniques with a large number of bubble samples. Two inlet conditions, controlled sized bubbles and uncontrolled sized bubbles, are conducted to study the characteristic behaviors of bubbles. With the uncontrolled sized bubbles, the average bubble size corresponding to the two-dimensional bed is found, and is not affected by decrease or increase of flow rates. With the controlled sized bubbles, dominant bubble breakup and coalescence flows are separately simulated to investigate bubble breakup and coalescence rates. Unique behavior of bubble size distributions for dominant bubble breakup and coalescence has been seen, and changes in bubble size distributions along axial direction are studied with median bubble size. Near the inlet the median changes rapidly due to the dominant bubble mechanism of either coalescence or breakup, and far away from the inlet the median reaches asymptotic value due to the balance of bubble breakup and coalescence. For both dominant breakup and coalescence flows, the asymptotic values are close to the average bubble size. Therefore, the average bubble size, resulting from the balance of bubble coalescence and breakup at far downstream, is irrespective of inlet flow conditions. (C) 2010 Elsevier Ltd. All rights reserved.