Quantitatively characterizing long-period fluctuations, which reflect the large-scale structure of a bubbly flow in a bubble column reactor, is essential to improving the reactor efficiency. We propose a newly developed method to extract the long-period fluctuations based on point-wise void fraction measurements. The validity of the new method is demonstrated via characterization of long-period fluctuations in a bubble column approximately 400 mm in diameter and 2000 mm in height. First, the characteristics of liquid-phase motion induced by a single bubble-swarm are described based on the results obtained via LDA measurements. The large-scale liquid-phase motion is characterized by an individual bubble cluster (i.e. a region with the same void fraction). We explain the premise of our new method based on these results. Second, we show that the bubbly flows in the bubble column can be divided into three regions (i.e. a time-spatially fluctuated region, a transition region, and a pseudo-homogenous region) based on differences in the distribution patterns of bubble diameters and velocities as well as those between time-series point-wise void fractions obtained from four-point simultaneous measurements. The spatial fluctuations fade out with height from the column bottom. Finally, analyzing the time-series point-wise void fractions measured via four-tip optical-fiber probe (F-TOP), we demonstrate that the long-period fluctuations can be extracted via waveform analysis. The characteristic spectrum pattern also fades out with height. The extracted long-period fluctuations are in good agreement with those obtained from the visualization results for the flows. (C) 2010 Elsevier B.V. All rights reserved.