Phase distributions were studied experimentally in fully developed laminar bubbly flow with non-uniform bubble sizes. A stereo-photographic method was designed to determine the three-dimensional position and size of each bubble in the flow field. The void fraction distributions were determined for every group bubble with approximately uniform size. The phase distributions for uniform bubble flows were also measured for analysis of the effect of the bubble size on the behaviour of the bubbly flows. The results show that if the average bubble diameter of a group of bubbles is less than about 3.5 mm, the void fraction distribution will peak near the pipe wall for both uniform and non-uniform bubble flows. For non-uniform bubble flows, large bubbles with diameters over about 3.7-3.8 mm create a void fraction profile with a peak near the pipe center. Bubbles with diameters from 3.5 to 3.8 mm tend to be distributed in a two-peak profile. The strong effect of bubble size on void fraction profiles is similar to that observed in previous investigations despite considerable differences in pipe size and working liquid properties. (C) 2001 Elsevier Science Ltd. All rights reserved.