Exposure of poly(dimethylsiloxane) (PDMS) to oxygen plasma creates a thin, stiff surface-modified layer that reaches a submicron depth. Due to a significant modulus mismatch between the stiff surface-modified layer and the compliant bulk PDMS the surface-modified layer forms intricate patterns of surface buckles when under compressive stress and nano-cracks when under tensile stress. It is desirable to be able to design patterns of nano-cracks, or at least to have an understanding of them. Among the properties necessary to do this are the thickness and elastic modulus of the surface-modified layer. Due to the very small length scale of the surface-modified layer, it is a significant challenge to measure these properties. In this proceedings paper, a two-step method is described for determining the thickness and elastic modulus of the surface-modified layer using the atomic force microscope (AFM). First, nanoindentation is performed from which the bending stiffness of the surface-modified layer is calculated. Second, the surface-modified layer thickness is determined by using phase imaging on the cross-section of oxidized PDMS to map the region of the relatively stiffer surface-modified layer.