Implantation of hollow fibers for blood oxygenation within a human vessel has been investigated for the last 15 years. Unfortunately, the combination of limited space inside the venous system and disadvantageous blood flow conditions has resulted until now in limited gas exchange performance of the investigated oxygenators. We are developing a highly integrated intravascular membrane oxygenator (HIMOX) characterized by a homogeneous disk-shaped fiber configuration. The main advantages are a larger fiber surface as well as favorable cross flow through the fibers compared with earlier designs. Fiber porosity represents an important constructive parameter and leads to a trade-off when dimensioning the bundles with the aim of maximum gas exchange at small anatomical size. Low porosity results in higher fiber surface as well as blood velocity. Both effects potentially enhance the gas exchange, but the associated increase of the pressure drop leads to a deformation of the fiber bundle and to a blood shunt. This fluid-structure interaction influences the gas exchange in a complex way. We investigated the influence of porosity on the gas exchange in the proposed fiber configuration in vitro. Bundle deformation was proven by comparing experimental data with a theoretical model. Highest oxygen exchange supplied by a single bundle was achieved at an intermediate porosity of 0.575. Moreover, specific oxygen exchange per fiber surface, which is an indicator of favorable flow conditions, increased with increasing fiber porosity. We achieved up to 450 ml O2 min–1 m–2, which is a promising result for intravascular membrane oxygenation.