An experimental study was performed in a 25.4 mm ID pipeline to evaluate the development of the bubble size distribution in the horizontal flow of an air-water system. As the air stream enters into the flowing water stream through a T-injector, it breaks into bubbles with a log-normal size distribution. Because of the small water velocity (1-3 m/s) and small initial bubble size, coalescence, not breakage, plays the dominant role in the present study. The effects of average water velocity, air volume fraction and air injector diameter on the initial bubble size distribution and its evolution along the length of the pipe in the coalescence dominant regime are investigated. At larger water velocities, the log-normal bubble size distributions are also maintained downstream of the injector. At smaller velocities, the distributions deviate slightly from the log-normal pattern. For all distributions, the value of the ratio d(99.8)/d(32) is about 2.2 and is fairly independent of average water velocity, pipe length, air volume fraction and air injector diameter. It is found that at large velocities of water, the prediction of d(max) through Levich's breakup theory agrees well with the experimental d(99.8) values for air volume fraction up to 0.003. (C) 2003 Elsevier Ltd. All rights reserved.