BACKGROUND: Selection of polymers for two-phase partitioning bioreactors (TPPBs) has been focused primarily on predicting a polymer's affinity for the target molecule. Although the extent to which a polymer absorbs a solute is important, the rate of uptake/release must be sufficiently rapid such that a TPPB is not mass transfer limited. This work focused on developing a guide to identify combinations of polymer diffusivities and diffusional path lengths that will ensure a TPPB is not limited by substrate delivery.