The invention relates to mark-recording systems, for example carbonless copying paper, in which a colourless chromogen is brought into contact with a chromogen-sensitizing agent in the presence of a liquid solvent for the chromogen, thereby producing a visible mark. In a system of the invention, the chromogen is a derivative of diphenylmethanol or diphenylmethylamine, the sensitizing agent is an acid clay, and the solvent comprises an aliphatic benzyl phthalate.
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