Abstract: The invention provides a method for the determination of the phase change characteristics of a hazardous material, the method comprising optically recording images of a sample of the material and objectively evaluating the images. The phase change characteristics generally relate to a solid/liquid or liquid/solid phase change, and the method finds particular application in the determination of the crystallisation points of hazardous materials, most particularly radioactive materials. The images are typically recorded by means of a camera, and evaluated using a computer, which is able to objectively determine the point at which a phase change is first seen to occur, and thereby provide accurate, reliable and objective data.