Changes in the structure and chemical composition of polyamide (PA) composite membrane surface were correlated with (a) changes in the coating conditions employed to prepare the membrane; and with (b) resultant changes in performance. Thin film composite (TFC) membranes were formed by the interfacial polymerization of water-soluble difunctional amine with an organic-soluble trifunctional cross-linking agent on top of a porous polysulfone (PS) support. For different sets of film-formation conditions, different performances were exhibited with respect to product water flux and salt rejection. The thickness of the ultra-thin barrier layer of PA which governs the membrane performance characteristics such as permeability and permselectivity, is found to vary under different coating conditions in the range of 0.10-0.30 mum. Accordingly, an inverse correlation exists between PA film thickness and membrane flux. The composite membranes were characterized by Attenuated total reflectance infrared (ATR-IR) spectroscopy technique which allowed us to identify the main functional groups of PA barrier layer and thereby estimate its thickness based on the calculated depth of penetration (d(p)) of infrared beam into the sample material and absorbance of the carbonyl-stretching characteristic band pertaining to amide linkage. Extensive characterization of a variety of TFC membranes could provide us greater certainty for drawing conclusions about structure-performance relationship.