The bubble size distribution and void fraction (epsilon(g)) (at two bulk liquid pool positions below the bulk liquid-foam interface and one lower foam phase position) in a continuous foam fractionation column containing ovalbumin were obtained using a photoelectric capillary probe. The bubble size and epsilon(g), data were gathered for different operating conditions (including the changes in the superficial gas velocity and feed flow rate) at a feed solution of pH 6.5 and used to calculate the specific area, a, of the bubbles. Thus, local enrichment (ER,), values of ovalbumin could be estimated and compared with directly obtained experimental results. The ER, results were also correlated with the bubble size and epsilon(g) to understand better the concentration mechanisms of foam fractionation. The high ER, in the lower foam phase was largely attributable to the abrupt increase in eg (from 0.25 to 0.75), or the a (from about 12 to 25 cm(2)/cm(3)) from the bulk liquid to the foam phase. These changes correspond with enhanced gravity drainage. With an increase in the superficial gas velocity, the bubble size increased and the a decreased in both the bulk liquid and lower foam phases, resulting in a decrease in the local experimentally determined enrichments at high superficial gas velocities. At intermediate feed flow rates, the bubble size reached the maximum. The epsilon(g) and a, on the other hand, were the largest for the largest feed flow rate. The ER1 in the lower foam phase was maximized at the lowest feed flow rate. It follows, therefore, that a alone is not sufficient to determine the magnitude of the ER1 in the foam phase.