Membrane distillation is an emerging thermal membrane technology for the separation of salts and other non-volatile inclusions from water streams. The process offers a solution for the treatment of concentrated solutions, which are not viable for reverse osmosis. However, only few studies focused on the optimal membrane properties and operational conditions in the high concentration regime. In this paper, membranes with variations in thickness, porosity and structure are experimentally investigated in direct contact membrane distillation (DCMD); in addition, the performance is simulated using the Dusty Gas Model. Operational conditions, including the temperature difference over the membrane, the flow velocity and the feed stream salinity up to saturation were varied. It was confirmed that for pure water, thinner membranes show higher fluxes, while energy efficiency is unaffected by membrane thickness. At higher salinities, an optimal membrane thickness depending on membrane parameters and process conditions exists. The optimal membrane thickness computed in this article ranges from 2 to 739 pm for concentrations of NaCI ranging from 0 up to 24 wt% and variations in bulk temperature difference and flow velocities for four different membrane structures. (C) 2015 Elsevier B.V. All rights reserved.