Attenuated total internal reflection Fourier transform infra-red (ATR-FT-IR) spectrum analysis is done to characterize a thin film composite (TFC) nanofiltration (NF) membrane. The chemical compositions of the skin layer (composite polyamide) and the support substrate (polysulfone) are evaluated. Using a high-resolution optical microscope, the thickness of the different layers of the composite membrane is measured. The deposition and penetration of crystal violet molecules on and inside the composite membrane during nanofiltration at different operating conditions are optically measured. The deposition and penetration as a function of location on the membrane are also measured. The experimental results are found to be consistent with the basic physics of the process. (c) 2006 Elsevier B.V. All rights reserved.