Two main scales of mixing can be considered inside a stirred tank: macromixing and micromixing. Macromixing is related to the tank size circulation and is responsible for bubble motion, surface aeration and tank homogenization. Meanwhile, micromixing is related to the small liquid eddies, responsible for the concentration gradients surrounding the bubbles, and it prevails around the impeller. Experimental results and empirical equations are used anti proposed to unveil the contribution of both mechanisms to the volumetric mass transfer coefficient, k(L)a. A model has been proposed to predict a mean k(L)a as a combination of both mechanisms. In case of consumption of oxygen due to chemical reactions, micromixing plays a dominant role. However, only by considering the macromixing contribution to the total k(L)a is it possible to explain the scale tip problems traditionally reported for stirred tanks due to the important effect that the hydrodynamics plays: removal of the superficial air liquid contact, baffles, stirrer type, etc. (C) 2008 Elsevier B.V. All rights reserved.