We describe time-of-flight secondary ion mass spectrometry (SIMS), depth profiling, and atomic force microscopy studies of nanoporous silica (NPS) ultralow-k (ULK) dielectric films after C4F8/90% Ar plasma etching. The goal of this work was to establish the role of materials chemistry and porosity on determining the character of plasma/material interactions. To this end, NPS materials with overall porosity ranging from 15% up to 50% were used. For comparison, identical studies were performed on organosilicate glass (OSG, porosity ≈7%) and SiO2. The etching time of the samples varied between 0 and 30 s. Our work clearly shows two distinct regimes of plasma/nanoporous material interactions: fluorinated compounds are formed upon the reaction of the plasma species with the NPS dielectrics, resulting in severe chemical changes of the pristine materials and causing an almost constant distribution of fluorine throughout the entire residual low-k layer. Surface roughness is also introduced as a function of the etching time, with the final morphology strongly increasing with NPS porosity. On the other hand, no surface roughness develops on OSG and SiO2 even after prolonged plasma treatments. No compounds directly related to the chemical modification of these dielectrics are observed while the deposition of a surface layer of fluorocarbon species characterizes the etching regime. The in-depth distribution of fluorine in SiO2 is restricted to the very near-surface region. F is present throughout the whole residual layer in OSG but its concentration is lower than in NPS bulk. Our results suggest a model where the overall porosity of the dielectric plays a dominant role in controlling the plasma/ULK surface inter- - action. Porosity influences the surface coverage of fluorocarbon compounds. This in turn affects the interaction of the plasma species with the dielectrics, and results in chemical modifications and surface coarsening of the materials. The threshold in the onset of the two interaction regimes is established for porosity values around 10%. It is possible that this porosity threshold signifies the onset of pore interconnectivity (open pores), which becomes important for the higher porosity values.