Abstract: A diazotype sheet is developable solely by heat, the sheet comprising in at least one layer a heat-softenable, organic solvent-soluble resin, an acid-stabilized diazonium salt, at least one azo-coupler compound capable of reacting to form a dye, and an acid neutralizing component immobilized either by encapsulation or by coating as a separate layer with or without a binder. The sheet has incorporated therein at least one plasticizer which is either liquid at room temperature (i.e., 20.degree. to 25.degree. C.) or fusible at a temperature between room temperature and the development temperature.