Quantification and control of the residual layer thickness is a critical challenge facing nanoimprint lithography. This thickness must be known to within a few nanometers, yet there are very few nondestructive measurement techniques capable of extracting such information. Here we describe a specular x-ray reflectivity technique that can be used to not only quantify the thickness of the residual layer with sub-nm resolution, but also to extract the pattern height, the line-to-space ratio, and relative linewidth variations as a function of the pattern height. This is illustrated through a series of imprints where the initial film thickness is varied. For films with sufficient resist material to fill the mold, complete pattern filling is observed and the residual layer thickness is directly proportional to the initial film thickness. When there is insufficient resist material in the film to completely fill the patterns in the mold, a finite residual layer thickness of approximately 50-100 angstrom is still observed. (c) 2005 American Vacuum Society.