We discuss optical reflectometry as a tool for studying adsorption from dilute solutions. In particular, we elaborate on the kind of reflectometry where the reflecting substrate carries a thin dielectric film on which the adsorption takes place. The role of this film is to enhance the sensitivity of the method by producing a suitable phase shift in the reflected beam. The experimental set-up, calibration procedure and some conditions for successful determination of the surface excess are described. By means of an optical model we then calculate sensitivity and linearity of the technique for various solvents and adsorbents and we determine the optimum thickness of the dielectric film and the most appropriate angle of incidence. We find that, provided such optimum conditions are chosen, the technique allows sensitive and accurate detection of the adsorbed mass per unit area in all cases where the refractive index of the adsorbate film is sufficiently different from that of the solvent. For a typical example (organic solutes in water), adsorbed amounts as low as 0.01 mg/m2 can be detected.