In the present work, we explore the underlying chemistry occurring on the molecular level during plasma processing of a variety of polymeric membranes, continuing our studies on plasma modification of polymeric membranes for permanent hydrophilicity. Characterization of gas-phase species as well as molecules generated at the membrane surface during plasma modification was performed using optical emission spectroscopy and mass spectrometry. Plasma-surface interactions of OH radicals were assessed with our direct, nonintrusive radical-imaging experiments based on laser-induced fluorescence. Results show that OH has a moderate reactivity of similar to 50% on all membranes studied, regardless of applied plasma power. Alternate reactor designs were employed to examine formation of reactive species and the effects of ions on the surface modification. Collectively, these experiments provide information on the chemical mechanisms responsible for surface modification and, ultimately, afford better process control for plasma treatments of polymers.