The corrosion rate of a metal immersed in a fluid medium is measured by transmission of a beam of radiation normally in the visible or near infra-red portion of the spectrum, through a thin film of the metal immersed in the medium. The film of the metal is suitably supported on a radiation-transmitting substrate such as a glass plate or slide. The corrosion rate can be determined by passing a radiation beam through the metal film sample using a twin beam system to compensate for instrument factors such as the absorbance by the fluid medium, the cell windows and the film-supporting substrate. As the thickness of the film decreases, the reduction in film thickness is determined by the increase in beam intensity, using a reference beam to compensate for the instrument factors.