A model for the mass- and heat-transfer phenomena in an osmotic evaporation process was developed based on the transfer of solvent from one aqueous solution to be concentrated to a second one separated by a macroporous hydrophobic membrane. The transfer is realized in vapor phase through the membrane porosity as a consequence of the gradient on water activity between both solutions. This technique has the major advantage to work at relatively low temperature and in nearly isothermal conditions, which is very useful to treat thermosensitive products such as fruits and vegetable juices, or other solutions from biotech industries. A series-resistance model for mass and heat transfer was developed considering four regions, which include the membrane layers and different boundary regions at interfaces. The high values of flow measured in a previous experimental work are compared with those obtained by simulation. In addition, the relative importance of various resistances to overall process performance is established. Basic mechanisms that help optimize membrane structural characteristics, plant design and scale-up are also discussed.