The four-point optical probe is applied in a bubble column with an air-water system to investigate the bubble properties (local gas holdup, velocity, chord length, specific interfacial area, and frequency) over a range of gas superficial velocities. Both bubbles moving upward and downward are recorded and measured as opposed to only upward bubbles measured and reported in other studies involving probes. The probe worked efficiently in both bubbly flow and highly churn-turbulent flow at very high superficial gas velocities. Bubble properties at the conditions of churn-turbulent flow are obtained and investigated for the first time. The changes in the bubble velocity distribution, bubble chord length distribution, and specific interfacial area with superficial gas velocity, sparger design, and with axial and radial positions in the column are discussed. (c) 2007 American Institute of Chemical Engineers.