Osmotic distillation (OD) is a concentration technique for aqueous mixtures based on porous hydrophobic membranes in contact on both sides with liquid phases at pressure lower than the pressure needed to displace the gas phase in the pores. The driving force for the water vapour diffusion through the gas phase immobilised within the membrane pores is sustained by an activity difference by using a hypertonic solution, typically concentrated brines, downstream the membrane. The mass transfer causes a cooling down of the feed and a warm up of the brine, as a consequence a temperature difference is created which reduces the effective driving force for mass transfer. This ＇thermal effect＇ is investigated both theoretically and experimentally, it is shown that the effect on the flux is substantial.