A comparison was made of the gas hold-up values obtained in an aerated mixing vessel of high aspect ratio by both visual measurement of the liquid level rise due to aeration, agitation and processing, and the residence time distribution (RTD) curves of the gas flowing in the vessel. For the description of the RTD curves, the one-dimensional dispersion model with Danckwerts' boundary conditions was adopted and the corresponding equations were solved analytically. Experiments have been performed with both coalescent and non-coalescent systems. It was found that gas hold-up values determined with these two methods are in very good agreement. This also provides additional support for the use of the proposed model.