Radiation sensitive compositions are image-wise exposed and developed to form an image which is then heated to improve its strength as a lithographic printing image or etch resist. The heating is carried out in the presence of a heat sensitive dye which undergoes a color change at a temperature of at least 180.degree. C. so as to obtain an indication of whether or not the heating has been adequate. The dye may be an integral component of the composition or it may be applied to the image, after development, in combination with a substance capable of forming a shield against contaminating residues produced during the heating step.