The most important scale-up parameters of aerated bioreactors are investigated in 42 and 340 L vessels, with water and various xanthan gum and carboxymethyl cellulose solutions. The study focuses mainly on mass transfer (k(L)a) measurements under various operating conditions. The relevance of existing correlations is discussed. The traditional viscosity-contribution approach appears unable to predict the changes in kLa during scale-up and an alternative formulation is proposed. The effect of rheology on power consumption and mixing time is in fair agreement with works published on this topic.