AbstractThe application of four‐point optical probe has been for the first time extended to a slurry system in a 10.2‐cm ID column. Bubble dynamics (local gas holdup, bubble chord length, bubble velocity, bubble frequency, and specific interfacial area) were investigated using an air–water‐catalyst system under atmospheric pressure. With an increase in solids loading, the local gas holdup, specific interfacial area, and bubble frequency decreased, while the bubble velocity changed slightly. Bubble chord length increased noticeably, and the bubble chord length distribution spread more widely at high solids loading. It has also been found that the probe orientation is important for an investigation using probes, especially in the wall region of the column. © 2008 American Institute of Chemical Engineers AIChE J, 2008