Bubble size is used to characterize not only bubble-specific interfacial area but also bubble coalescence in a foam column. The bubble size distributions were obtained in a continuous foam fractionation process for concentrating ovalbumin using a developed photoelectric probe. When the continuous process reached steady state, the bubble size distribution pattern remained stable. Bubble size distribution data above (+ 1 cm) or below (- 1 cm) the bulk liquid-foam interface showed symmetry along the diameter of the column (14 cm ID). The bubble size distribution was affected by the column wall. The nearly constant protein concentration distribution across the column cross-section indicated that the bubble flow distribution approached a flat profile across the column. A log-normal bubble distribution pattern best fit the weighted ran-e of bubbles in the column at column lengths above and below the liquid-foam interface. These observations may prove to be useful in understanding the mechanisms underlying the foam fractionation of proteins. (C) 2003 Elsevier Science (USA). All rights reserved.