Oxygen mass transfer in sparged stirred tank bioreactors has been studied. The rate of oxygen mass transfer into a culture in a bioreactor is affected by operational conditions and geometrical parameters as well as the physicochemical properties of the medium (nutrients, substances excreted by the micro-organism, and surface active agents that are often added to the medium) and the presence of the micro-organism. Thus, oxygen mass transfer coefficient values in fermentation broths often differ substantially from values estimated for simple aqueous solutions. The influence of liquid phase physicochemical properties on kLa must be divided into the influence on kL and a, because they are affected in different ways. The presence of micro-organisms (cells, bacteria, or yeasts) can affect the mass transfer rate, and thus kLa values, due to the consumption of oxygen for both cell growth and metabolite production. In this work, theoretical equations for kLa prediction, developed for sparged and stirred tanks, taking into account the possible oxygen mass transfer enhancement due to the consumption by biochemical reactions, are proposed. The estimation of kLa is carried out taking into account a strong increase of viscosity broth, changes in surface tension and different oxygen uptake rates (OURs), and the biological enhancement factor, E, is also estimated. These different operational conditions and changes in several variables are performed using different systems and cultures (xanthan aqueous solutions, xanthan production cultures by Xanthomonas campestris, sophorolipids production by Candida bombicola, etc.). Experimental and theoretical results are presented and compared, with very good results. © 2005 Wiley Periodicals, Inc.