The complex structure of the active aromatic polyamide (APA) layer of reverse osmosis membranes needs to be precisely described for understanding and predicting solute rejection. A commercial reverse osmosis membrane (CPA2, Hydranautics) was chosen as a case study, and home-made APA films were prepared by free-standing interfacial polymerization of trimesoyl chloride (TMC) and m-phenylenediamine (MPD) at different temperatures to obtain different film structures. Their morphology, thickness, void fraction, density, and chemical composition were characterized by scanning electron microscopy (SEM), atomic force microscopy (AFM), profilometry, ellipsometry, dynamic vapor sorption (DVS), and X-ray photoelectron spectroscopy (XPS). All samples have a multilevel topological structure as a dense base upon which generate a valley-ridge structure for CPA2 and a chimneylike structure for synthesized samples, whose thicknesses increase with temperature. An average void fraction of 35% was deduced for the APA from CPA2, allowing the calculation of a dry density of its dense phase of 1.24 g.cm(-3), which is rarely investigated for commercial membranes. New chemical descriptors considering chain heads, tails, and branches were introduced to improve the interpretation of chemical composition results. Being more realistic than the cross-linking degree, they suggested for the CPA2 active layer a back surface chemically less cross-linked than the top surface.