In the selection of ultra low k materials, process compatibility is a very important factor. Plasma processing plays a critical role in enhanced interconnect integration. It is therefore important to study plasma interaction with the ultra low k materials and its effects on the structure and property of these materials. X-ray reflectivity (XRR) measurement can be used to measure film thickness, density and interface roughness, which are important parameters to check for after plasma treatments. In the current study, porous SiLK (p-SiLK) was treated with various plasmas, such as O2, O2/N2, H2/N2, CH2F2/Ar and CF4/O2. XRR results indicate that the density of the p-SiLK films remains unchanged after various plasma treatments. Surface roughening occurs during the plasma treatments, accompanied by the decrease in film thickness. Plasma-induced surface roughening was also observed using atomic force microscope (AFM). Such roughening is more severe for plasma treatments using oxygen-containing plasmas. FTIR analysis indicates that the chemical structure of the p-SiLK films is not significantly affected by plasma treatment. It is reasonable to conclude that oxidation of the surface plays a major role in the plasma-induced change in surface roughness and film thickness.