Free-radical photopolymerization performed within PDMS microfluidic devices is now used for a variety of applications. We propose, through model and experiment, that atmospheric oxygen diffusing in through the porous PDMS is responsible for the presence, under UV light, of a thin, un-cross-linked film of oligomer abutting the walls of an all-PDMS device. After the advent of light exposure, an induction time τi is required before the oxygen present in the oligomer is depleted, and cross-linking reactions can begin. A polymerized structure then grows from the center of the device outward, increasing sharply in height with time and leaving only a thin un-cross-linked film of thickness, δi,c, close to the walls where oxygen can penetrate. Under suitable simplification of the reaction−diffusion model developed, scaling relationships were obtained for τi (∼Da−1) and δi,c (∼Da−1/2) as a function of a Damkohler number, Da. The relationships were successfully verified by comparison with both the full solution and ...