ROCSOL is a solar reactor for the study of the melting and freezing behaviour of materials up to 3000°C under oxidizing atmosphere. It is associated to a 2 kW horizontal axis solar furnace. The incident solar heat flux on the parabola may be regulated by means of a primary shutter on the path of the parallel solar beams between the heliostat and the concentrator to control the duration of the freezing plateau. A rotating cylindrical reactor is located at the focus of the concentrator. The cell containing the material is a water-cooled rotating cavity (diameter 0.03 m, length 0.04 m) with an aperture of 0.014 m in diameter. The melting of the material occurs in a self crucible insuring high purity treatment. The concentrated solar flux can be quickly switched off by a water-cooled shutter located on a fast-moving mechanism. This movable shutter have been designed to eliminate completely the solar heat flux in less than 0.1 second. The temperature of the material is measured by optical pyrometers located behind the parabola and focused at normal incidence through the aperture of the reactor inside the sample cavity.