This experimental study describes how the height of a 2D gas-solid fluidized bed can be measured by computer vision. A CCD camera is laterally positioned so that it views the bed's top surface and scans the interface. Then, through pattern recognition, the camera finds the most horizontal segment marking the freeboard-fluidized bed interface. Here, we illustrate this method using 420-500 mu m glass ballotini and examine the behaviour of the bed for different quantities of fluidized material. The data obtained were used to determine the relationship between the average height of the bed and gas flow. Minimum fluidization velocity and bed expansion were experimentally obtained. The histograms of the bed height measurements indicate that for a given gas flow, this variable shows a most likely value. When progressively varying the gas inlet to the bed the transition between packed and fluidized bed states revealed four different volume change patterns that could be attributed to the bubbles entering and leaving the bed. (c) 2013 Elsevier Ltd. All rights reserved.