Oxygen mass transfer in a membrane blood oxygenator (MBO) surrogate system has been addressed in this work. It consists of a slit for water circulation as a surrogate blood flow channel and a constant pressure oxygen chamber separated by an integral asymmetric hemocompatible polyurethane-based membrane. The oxygenated stream enters a well-mixed reservoir of constant volume, V, for the oxygen average concentration, , measurement as a function of time, t. In a range of short times, the linearity of vs. t allows the direct determination of the permeation fluxes JO2, with no recourse to dimensionless correlations for the determination of mass-transfer coefficients. The experimental fluxes are in very good agreement with the predictions based in unidimensional axial convection and unidimensional transversal diffusion. This custom-made benchmark system allows the optimization of the flow and oxygen mass transfer for the design of a novel flat-sheet MBO. (c) 2018 American Institute of Chemical Engineers AIChE J, 64: 3756-3763, 2018