The diffusion of Si in thin films of photoresist during silylation was investigated using Rutherford backscattering spectroscopy (RBS) and the chemical reaction was studied by X-ray photoemission spectroscopy (XPS). Initially, Si diffuses very rapidly, then it slows down as the silylation time increases. Almost all the Si needed to form a stable cross-linked matrix is achieved within 5 min of silylation. With reactive ion etching (RIE) the etch rate of the silylated photoresist depends on the silylation time and whether the sample is blown dry, air-dried or rinsed immediately after the silylation process. A long silylation time results in the formation of a thin layer of SiO2 on the resist, which increases the RIE resistance. © 1990.