In this work, the impact of the spacer features (interpillar distance and shape) on the membrane deflection, pressure difference across the membrane, and mass transfer was studied for a flat membrane microcontactor. It was demonstrated that decreasing the interpillar distance between the circular pillars reduced membrane deflection considerably. As a consequence, also the pressure difference across the membrane decreased, which lowers the possibility of breakthrough occurring. However, this was only the case until an interpillar distance of 836 mu m was reached. Decreasing the interpillar distance further, the pressure difference across the membrane again increased. This was due to the smaller gap between the pillars, resulting in a higher hydraulic resistance. However, it was demonstrated that with the use of a channel filled with radially elongated (diamond shaped) pillars and wedges the pressure difference across the membrane remained minimal at small interpillar distances (i.e. 305 mu m). Finally, the effect of the membrane deflection on the mass transfer was studied. It was shown that membrane deflection can have either a positive or negative impact on the mass transfer and solvent inside the pores instead of the feed is not always beneficial when membrane deflection is taken into account. Therefore, design rules for the spacer geometry are deduced, which allows maximizing mass transport. (C) 2016 Elsevier B.V. All rights reserved.