Integral asymmetric bi-soft segment poly (ester urethane urea) membranes are casted through a novel procedure where a reaction is introduced in the casting solutions preparation step of the phase inversion technique. Two reactive pre-polymers, the polypropylene oxide-based with three terminal isocyanate groups and the polycaprolactone diol with two terminal hydroxyl groups, are dissolved in a single solvent, dimethyl formamide and in a solvent mixture of dimethyl formamide (DMF)/diethyl ether (DEE) and the polymerization reaction is carried out in a homogeneous phase. The weight ratio of DMF/DEE is varied over 11,5 and 3. The membrane casting is carried out at solvent evaporation times of 30 s, 60 s and 90s. For all the membranes prepared, the cross-sections of the scanning electron microscopy (SEM) images display asymmetric structures dependent on the casting solutions composition and on the casting evaporation times. The carbon dioxide and oxygen volumetric fluxes measured at pressures ranging from 76 cmHg to 380 cmHg were correlated to the active layer thickness of the asymmetric membranes. The achievement of increasing the O-2 and the CO2 permeation rates is the result of the casting of poly (ester urethane urea) asymmetric membranes with the minimization of the thickness of the dense layers. This is coupled with the introduction of the reactive caprolactone pre-polymer that enhances hemocompatibility. These are two strong assets for membrane blood oxygenators. (C) 2011 Elsevier B.V. All rights reserved.