A process for preparing a photothermographic material with increased photosensitivity, the photothermographic material comprising a support and a photo-addressable thermally developable element, exclusive of a compound R--S(M).sub.n wherein R is an aliphatic hydrocarbon, aryl or heterocyclic group, M is a hydrogen atom, or cation, and letter n is a number determined so as to render the molecule neutral and also exclusive of a compound capable of releasing a mobile dye corresponding to or inversely corresponding to the reduction of silver halide to silver at elevated temperatures, the photo-addressable thermally developable element containing a photosensitive agent in catalytic association with a light-insensitive organic silver salt, a reducing agent for the light-insensitive organic silver salt in thermal working relationship therewith and a binder, comprising the steps of: (i) increasing the photosensitivity of a photosensitive silver halide by chemical sensitisation with a chemical sensitising merocyanine dye containing a thione group in a non-oxidative aqueous medium, optionally in the presence of the light-insensitive organic silver salt, thereby producing the photosensitive agent; (ii) coating the support with one or more aqueous solutions or dispersions together containing the light-insensitive organic silver salt, the photosensitive agent, the reducing agent and the binder; (iii) drying the coating or coatings thereby producing the photo-addressable thermally developable element.