Bubble-bubble interactions in a wall-sliding bubble swarm are investigated experimentally using a particle tracking velocimetry (PTV) technique. Firstly, the drag coefficient of a single wall-sliding bubble is measured, in which the distance between the wall and the bubble interface is much shorter than the average bubble diameter. Secondly, the probability distribution of the nearest bubble around individual bubbles in the bubble swarm is detected in the range from Re = 1 to around 20. Two kinds of statistical sampling techniques are used in order to identify the two-dimensional structure of the bubble-bubble interactions. A local modification of the drag coefficient in the swarm is obtained by calculating the interactive velocity of an individual bubble. Furthermore, the transition of a bubble's arrangement in the swarm is discussed by computing the relative velocity vector. The feature of the interaction patterns obtained by these techniques is classified based on Reynolds number. (C) 2004 Elsevier Ltd. All rights reserved.