Reflectometry is an optical technique for the determination of the adsorption of molecules from solution onto a macroscopically flat substrate. Fixed angle reflectometry measures the reflectivity at the Brewster angle on a flat surface. It is usually combined with a stagnation point flow cell. A linearly polarized light beam is reflected from the adsorbing surface. The reflected beam is split into its parallel and perpendicular components (p- and s-polarized respectively) of which the intensities are measured continuously (Ip and Is respectively). Upon adsorption of molecules at the surface the ratio of these intensities changes. From this change the adsorbed amount can be obtained after calibration of the system.