Infrared spectroscopy has been combined with oxygen sputter etching to produce a technique for determining depth profiles of the chemical and physical structure of organic thin films. This is achieved by measuring the infrared reflection‐absorption spectrum of the organic film, supported on a metallized substrate, as the film is sputter etched in an rf oxygen plasma. The thickness of the damaged layer produced on the film as a result of this etching is small, 2–5 nm, and comes to equilibrium very quickly. Subtraction of appropriate spectra then allows the determination of the infrared absorption spectra of thin sections (10–20 nm thick) throughout the depth of 100–200‐nm‐thick films. Results will be presented on infrared depth profiling to characterize the interpenetration of thin films of oligomers and polymers and to measure the nature, extent, and depth of laser surface modification of polymers.