An optical method (real-time holographic interferometry) has been used to visualize concentration changes in the vicinity of the membrane surface during the dead-end reverse osmosis of salt solutions. This interferometric technique is based on the fact that changes in refractive index, which are associated with changes in concentration, can be visualized as interference fringes. Reverse osmosis experiments with NaCl and Na 2 SO 4 solutions with feed concentration in the range of 1-7 kg/m 3 at a constant pressure of 600 kPa have been conducted. Interferograms obtained under different experimental conditions, as well as permeate flux and membrane rejection, are presented. Concentration profiles in the concentration polarization layer have been determined from these interferograms and compared with those calculated using the mixed convection-diffusion and osmotic pressure theory or Fick's second law of diffusion, depending on whether the profiles correspond to the development or the disappearance of the layer. The reasonable agreement obtained between experimental and calculated results seems to support the validity of the mathematical models proposed in the range of the experimental conditions studied.