The gas hold-up in a 2D bubble column is modeled by using a 3D gas hold-up model. The effect of the scale of 2D bubble columns on several parameters, for instance, transition gas hold-up, transition gas velocity, and bubble rise velocities, is investigated and related to 3D bubble columns. By adapting the rise velocity of the large bubbles of an existing 3D bubble column model (R. Krishna et al., 2001), the gas hold-up in both the homogeneous and the heterogeneous regime can be described satisfactorily. By adjusting the transition points only, it is also possible to describe the gas hold-up in systems contg. small amts. of carbon particles and electrolyte. The smallest dimension of the 2D slurry bubble column, the column thickness, affects the location of the regime transition point. In the heterogeneous regime, however, it is only the largest column dimension, the column width, that affects the gas hold-up. These observations together enable proper 2D/3D bubble column comparison in future studies.