Microcapsules for copying material are prepared by dissolving a diazonium salt and shell-forming reactants in a non-aqueous solvent having a boiling point of from 40 to 95°C at atmospheric pressure, e.g. an ester, emulsify­ing the solution in a hydrophilic protective coloid solution, heating to cause shell-formation around the resultant oil droplets, and preferably under reduced pressure to remove the non-aqueous solvent. A coupling component and a basic substance are mixed with the microcapsules and together coated on a support, e.g. of paper or a synthetic resin film, optionally transparent, and dried to form heat-develop­able copying material in which the non-aqueous solvent is no longer detectable.
Preferred basic substances are guanidines, and couplers can be active methylene compounds. A low-boiling co-solvent may be added to the organic solvent to obtain a transparent copying material. The sensitive layer may be composed of several sub-layers; by using several diazo compounds multi-colour material is obtained.
In use, the material is image-wise exposed to light through an original whereby the exposed area is fixed, and overall exposed to develop the coloured image. Good, stable images are obtained.