Aqueous Two-Phase Extraction (ATPE) represents a promising unit operation for downstream processing of biological products. It is known to provide gentle extraction conditions due to the aqueous phases' high biocompatibility. However, some aqueous phases' physicochemical properties can lead to long phase separation times that might be limiting for classical ATPE setups. The Tunable Aqueous Polymer-Phase Impregnated Resins (TAPPIR (R))-Technology has been presented as novel approach to eliminate phase separation. One aqueous phase of the system is immobilized inside porous solids whereas the second aqueous phase represents the surrounding bulk phase creating the interphase for mass transfer between the two phases. In the present work, the application of the TAPPIR (R)-Technology for the separation of proteins using a polyethylene glycol 4000/sodium citrate aqueous two-phase system is shown. Lysozyme and myoglobin are separated due to their different partitioning behavior dependent on NaCl content. By variation of the porous solids' properties like solid material, particle and pore size the influence on protein partitioning is investigated. As result, the same partitioning levels can be reached for the TAPPIR (R)-Technology as for classical ATPE mixer/settler experiments offering new alternative concepts for ATPE. (C) 2014 Elsevier B.V. All rights reserved.