Bubble-free aeration is applied in animal cell culture processes for oxygenation to avoid shear stress and foaming associated with direct sparging into the culture. Oxygen mass transfer across PTFE-membranes was determined in three different reactor systems that may be applied to animal cell cultures. Oxygen was supplied on the aeration side of the membrane by air- or oxygen-saturated aqueous liquids, or directly using air or pure oxygen, By using liquids, the bubble-free aeration may be combined with an ammonia removal system, using the same membranes and described in an earlier publication. By providing oxygen directly in gaseous form, the use of PTFE-tubing enabled a reduction in the mass transfer surface by a factor of two or three, in comparison to aeration with silicone tubing. PTFE-membranes were successfully used to control dissolved oxygen concentrations in cultures of different animal cell lines.