The invention relates to a heat-sensitive recording material, comprising a substrate paper having a heat-sensitive recording layer arranged on one side. A particulate, water-insoluble azo dye and/or planchettes in which a particulate, water-insoluble azo dye is dispersed are embedded in the substrate paper. Alternatively or additionally, a particulate, water-insoluble azo dye is dispersed in the heat-sensitive recording layer. If the paper product is treated with organic solvents in the course of a manipulation attempt, the dye particles are dissolved. The recording material is intensely discolored, and the manipulation attempt is irreversibly indicated.