A security paper authenticating system comprises, in combination, a security paper carrying both starch and an iodate salt, typically potassium iodate, and an authenticating composition comprising an acidic solution of an iodide salt, typically potassium iodide, the system being such that on applying the authenticating composition to authentic security paper, as by a pen, brush or stamp pad, iodine is generated and a characteristic starch-iodine coloration is produced. The authenticating composition is preferably aqueous or part-aqueous, and is preferably made acidic by means of a weak organic acid such as tartaric acid. The authenticating composition preferably also contains an antioxidant such as ascorbic acid. The invention extends to the paper and the authenticating composition individually.