An image processing technique was used to study dominant bubble mechanisms in a two-dimensional packed-bed at pore level under the bubbly flow regime. Bubble breakup and coalescence were identified as dominant mechanisms using a large number of image samples. Two types of coalescence mechanisms were identified that occur due to compression and deceleration associated with the bubbles and three breakup mechanisms were identified that are result of liquid shear force, bubble acceleration, and bubble impact. Data on various two-phase parameters, such as local void fraction, bubble velocity, size, number, and shape were obtained from the images. Results indicated that when a flow regime changed from bubbly to either trickling or pulsing flow, the number of average sized bubbles significantly decreased and the shape of the majority of the bubbles was no longer spherical. Although a mean bubble velocity of all sized bubbles was uniform for given gas and liquid superficial velocities, individual bubble velocities were quite different depending on the bubble location in the pore. The present bubble size distributions were compared with previous studies and the results on bubble size are in general agreement. (C) 2009 Elsevier Ltd. All rights reserved.