Microstructured polymeric films are fabricated by a novel replication method. A polymer solution is applied and contained between two substrates, of which at least one is a patterned PDMS mold. The ensemble is then put in an atmosphere containing water vapor, which diffuses through the PDMS. The absorption of water into the polymer solution causes the precipitation (phase separation) of the polymer while in contact with the microstructured molds. The thickness of the PDMS slab can be exploited to tune the water vapor transport and hence the phase separation kinetics and resulting polymer morphology. Removal of excess polymer solution from between two PDMS slabs, followed by vapor induced phase separation, can also result in microperforated polymer films with great control over the dimensions.