The synthesis of polyamide films used as active layers in reverse osmosis membranes was studied by in-situ diffuse reflectance spectroscopy, Rutherford backscattering spectrometry (RBS), and atomic force microscopy (AFM). Aromatic polyamide layers were formed by interfacial polymerization on porous polysulfone supports using varying concentrations of m-phenylenediamine (MPD) in water of 0.1-100 g/L with a fixed concentration of trimesoyl chloride (TMC) in hexane of 1 g/L and varying TMC concentrations of 0.1-10 g/L with a fixed MPD concentration of 20 g/L. Polyamide growth dynamics were monitored in real-time by diffuse optical reflectance at lambda=329 nm. A relationship was developed between diffuse reflectance and polyamide thickness. The diffuse reflectance data show that similar to 50% of the polyamide thickness is produced in <2 s for all TMC concentrations studied and for MPD concentrations > 2 g/L. All studied concentrations of TMC at a fixed 20 g/L MPD concentration produced a polyamide thickness of approximate to 120 nm. Polyamide thickness increased from approximate to 10 to 110 nm with increasing concentration of MPD at 1 g/L TMC. The roughness measured by AFM increased with increasing MPD concentration but decreased with increasing TMC concentration. At MPD concentrations <0.5 g/L, polyamide does not grow on top of the polysulfone. (C) 2012 Elsevier B.V. All rights reserved.