A new model for determining bubble size distributions in bubble columns by the dynamic gas disengagement (DGD) technique is developed. It is based on an idea of non-uniform steady state dirstribution of bubble dispersion. Interpreting the axial nan-uniformity, this model gives axial gas holdup distributions. If assuming an axially homogeneous dispersion, a radial gas holdup distribution can be obtained, The Sauter mean diameters or specific interfacial areas for several systems are estimated by the technique, The results for an air-water system agree with those measured by a five-point conductivity probe technique. The obtained axial gas holdup distributions agree well with reported measurements and the radial gas holdup distributions are also reasonable.