This paper presents the experimental study and analysis performed in order to better understand the link between activated sludge properties, rheological behaviour and oxygen transfer. The experimental set-up consists of a bubble column of 0.3 m3 continuously fed with activated sludge and a capillary rheometer, installed in two different wastewater treatment plants: a conventional activated sludge plant and a membrane bioreactor. In the bubble column, equipped with a fine bubble diffuser, the overall gas hold-up (is an element of(G)) and volumetric oxygen transfer coefficients (k(L)a) were measured. A fraction of the column outflow was sent to the capillary rheometer where the activated sludge rheological behaviour was investigated. Several properties of the studied activated sludge were characterised (MLSS, MLVSS, soluble COD, surfactants, surface tension, soluble cations) and their impact on rheology and oxygen transfer was examined. The experimental data showed that the parameters K and n, from the Ostwald-de Waele rheological model, were strongly related to the suspended solids concentration (in terms of MLSS or MVLSS). An increase in kLa was observed as the superficial gas velocity (U-G) was increased. Compared to clean water, the kLa coefficient was lower in activated sludge and still reduced with an increase of the MLSS concentration. This reduction could be partially attributed to a lower gas holdup (epsilon(G)) associated with an increase in the sludge apparent viscosity (mu(app)) leading to a reduction of the specific interfacial area (a). Subsequently, an estimation of the mean shear rate exerted by the bubble swarm in the column allowed to evaluate the sludge apparent viscosity (mu(app)) of the mixed liquor at a given superficial gas velocity and MLSS concentration. Finally an empirical correlation linking kLa to the superficial gas velocity (U-G) and the sludge apparent viscosity was obtained for both types of sludge. The good agreement between the experimental and the fitted data suggests that kLa can be estimated from the superficial gas velocity and the rheological behaviour. (C) 2015 Elsevier Ltd. All rights reserved.