Microfluidic devices can be molded easily from PDMS using soft lithography. However, the softness of the resulting microchannels makes it difficult to photolithographically pattern their surface properties, as is needed for applications such as double emulsification. We introduce a new patterning method for PDMS devices, using integrated oxygen reservoirs fabricated simultaneously with the microfluidic channels, which serve as "chemo-masks". Oxygen diffuses through the PDMS to the nearby channel segments and there inhibits functional polymer growth; by placement of the chemo-masks, we thus control the polymerization pattern. This patterning method is simple, scalable, and compatible with a variety of surface chemistries.