Measurements of the corrosion of steel components, such as nuts and bolts, in nuclear reactors present special problems because of the restricted access and the hot ( approximately 100 degrees C), radioactive (10 rad h^-1) environment which exists even during a reactor maintenance period. To operate in such conditions a small self-contained instrument package has been developed which can be continuously cooled and remotely controlled. It makes use of the fact that when the beam from a high power ruby laser is focused on to a point on the surface of the corrosion layer a certain depth of the layer is penetrated. When this penetration extends down to the bare metal the surface reflectivity increases substantially, and it has been found that the depth of corrosion at any point can be measured accurately by monitoring the reflectivity of the probed area using a low-power gas laser. The reflectivity of the corroded surface to the gas laser beam has been shown to have a characteristic variation with the number of high-power ruby laser pulses fired and rises sharply when the corrosion layer is penetrated. By comparing the total energy required to penetrate the corrosion layer with a calibration curve obtained on samples with known corrosion depths, the depth of corrosion on the steel component can be determined.