The use of selectively silylated resists to facilitate top surface imaging offers the potential for nanoscale lithography using both deep ultra violet (DUV) illumination and electron beam techniques. In this process, an exposure-generated crosslinking prevents silicon incorporation from silylating agents. In the non-crosslinked regions, a silylation agent reacts with OH groups in the resist to form silicon-oxygen bonds. During subsequent dry development in an oxygen plasma, the incorporated silicon attracts oxygen to form silicon dioxide, protecting the resist underneath. The adjacent un-silylated resist erodes (develops) anisotropically 25–100 times faster than the silylation-protected resist. The key to this process is the chemical formation of a silylated region at the top of the resist. A challenge associated with the silylation process, however, is lateral swelling of the silylated layer, which can lead to difficult dimensional control. The amount of swelling depends on the amount of incorporated silicon. Therefore, the uniformity and repeatability of the silylation process must be controlled. This paper will describe how spectroscopic ellipsometry has been used to characterize and monitor the resist silylation process in a non-destructive manner. The simultaneous characterization of the thicknesses and optical properties of a series of silylated resists will be presented. Optical metrology of the thickness of the silylated resists will be correlated with SEM cross-sectional analyses, and process uniformity will be quantified using 49-site wafer maps.