The gas disengagement technique is discussed in detail and is applied in a two-dimensional bubble column and slurry bubble column to obtain the bubble size distribution. Flow characteristics including velocities of each phase and dispersed phase size distribution during the gas disengagement process are obtained by using an advanced image analysis tool including a particle image velocimetry technique. It is shown that in the coalesced bubble regime, the bubble and flow characteristics during the gas disengagement process are complex, i.e., there are strong bubble-bubble interactions as well as interactions between the liquid flow and bubbles. Several different analogies of the disengagement processes assuming a bimodal bubble size distribution are discussed in terms of the behavior of large and small bubbles and their influence over one another. The small bubbles, which disengage within the fast bubble flow region, must be taken into account to evaluate the bimodal bubble size distribution. The applicability of the gas disengagement technique to obtain a multimodal bubble size distribution is also elaborated upon.