A crucial issue in the characterisation of gas-liquid stirred systems is the choice of a suitable model to describe the fluid dynamic features of both the liquid and gas phases. In spite of increasing attention to CFD tools, semi-empirical models are still extremely useful to interpret experimental data and to provide an insight into the physics of these systems with an affordable effort. Two types of problems arise in connection with the use of a model like plug flow with dispersion (PFD) for describing the gas phase with viscous liquids and, to a more limited extent, non-coalescent systems: inaccurate K(L)a data (when measured with dynamic techniques) and unrealistic Peclet numbers (this last being the parameter of the PDF model). Both problems can be attributed to the assumption of a single, unstructured phase for the ensemble of the gas bubbles. In the present paper, a two gas fraction model was introduced and used to interpret the experimental gas RTD curves in low to moderately viscous solutions in vessels stirred with triple impellers of two types (Rushton turbines and BT-6 impellers). Water, sulphate (non-coalescent) and (moderately) viscous PVP solutions were used. Two fractions of the gas hold-up were considered: large bubbles, whose behaviour was described with the usual PFD model, and small bubbles that were considered as perfectly mixed. The main parameters of the model are the Peclet number of the large bubbles fraction and the volumetric fraction of the large bubbles. The model proved capable to interpret the experimental RTD curves of the gas phase obtained after a pulse disturbance at the inlet. A comparison of the estimated volume fractions of small-size gas bubbles with experimental transient hold-up measurements was also carried out and good agreement was obtained. This study suggests the existence of a critical bubble size, which discriminates between the mixed and the plug flow behaviour of the small and large bubbles. The action of an antifoam agent on the hold-up structure was also studied for a single condition. (c) 2004 Elsevier Ltd. All rights reserved.