Abstract: This invention relates to a solution for detecting counterfeit paper currency. The solution contains iodine at a concentration of from 0.005 to 3 grams/liter and one or more solvents selected from the groups consisting of: alcohols, ketones, polyalcohols, esters, ethers, or mixtures thereof of solvents belonging to the same group or to several groups of these solvents. The solvent, if miscible with water, contains distilled water in a ratio of from 99:1 to 1:99 parts by volume. If not miscible in water, the solvent is saturated with distilled water at a temperature of 20.degree. C. The solution is preferably contained in ballpoint pens, fountain pens or other similar dispensers which are then used to countersign the paper currency. A counterfeit currency will cause a visible line to appear on the currency, in about one minute. However, in the case of genuine paper currency, the drawn line of solution does not change color and disappears completely from the countersigned currency.