Experiments were conducted in a 0.25 m diameter bubble column to investigate the effect of medium composition on oxygen transfer rate. Aqueous solutions, the composition of which mimics a mammalian cell culture medium, are used. The effect on oxygen transfer rate of additives used to protect cells against local hydrodynamic stresses induced by bubble coalescence and bursting is addressed, in the range of operating conditions (aeration rates) met in animal cell cultures. The mass transfer coefficient, the liquid viscosity and surface tension, and the bubble size distribution are measured as a function of liquid composition and of gas superficial velocity, allowing to decouple the effects of the different additive on k(L) and on a.