Although microorganism respiration inhibition by sodium azide (NaN3) is used in some studies to identify activated sludge adsorption capacity, little is known about the effect of this compound on the suspension properties. In this study we have investigated the effect of NaN3 addition on both volumetric oxygen mass transfer coefficient and rheology of activated sludge (AS) suspensions in a 1.9 L bioreactor. The rheological properties (shear thinning one) of AS suspensions with and without NaN3 addition are measured in situ (triphasic conditions). It appears that NaN3 addition leads to a deflocculation of AS suspensions and thus a decrease in apparent viscosity. A small amount of suspended solids was added in order to obtain identical apparent viscosities (under 1.2 or 46.3 s−1) for AS suspensions with and without NaN3 addition. KLa values were then measured in both respiring and non-respiring suspensions for different air flow rates (2, 3 or 4 L/min) and under low or high mechanical shear rate (1.2 or 46.3 s−1). Results show that under high mechanical shear rate, the respiration state for a given air flow rate does not impact the KLa values. On the contrary, under low mechanical shear rate, NaN3 addition induces an increase of KLa values in comparison with those obtained with the respiring biomass. This effect, for a same apparent viscosity, is attributed to the deflocculation observed in the presence of NaN3. Indeed, AS with and without NaN3 addition used for the KLa measurements induce a modification of the floc internal structure, corresponding to smaller floc size in the case of NaN3 addition.