The requirement of nanometer dimensional control in photolithographic patterning underlies the future of emerging technologies, including next-generation semiconductors, nanofluids, photonics, and microelectromechanical systems. For chemically amplified resists, dimensional control is mediated by the diffusion and reaction of photogenerated acids within a polymer-based photoresist matrix. The complex nature of the combined processes of reaction and diffusion prohibit the routine measurement of this phenomenon. Using small-angle neutron scattering, we have measured the form of the diffusion-reaction path of a photogenerated acid within a model photoresist matrix with a labeled protection group on the polymer side group. During the deprotection reaction, changes in the scattering form factor result from the shape and form of the deprotected regions. The individual volumes or blobs of reacted material are diffuse, with a fuzzy boundary between the reacted and unreacted regions. The impact of these results on the pattern quality is also discussed. (C) 2004 Wiley Periodicals, Inc.