The so-called "counterfeit detection pens" are not always reliable for detecting counterfeit notes because they only detect natural fibers or starch in the paper. If a counterfeiter uses paper with natural fibers or has bleached an authentic note and used that paper to overprint a higher face value on it, the pens will offer no protection. [EDIT]============================================================= It does this using a known reaction between Iodine and starch, called an ‘Iodine Test’. The Iodine Test is a standard chemical method of detecting starch. Simply stated Iodine reacts with starch to produce a deep blue-black color. The test solution in the DriMark pen is naturally a light golden-brown or yellowish color, but when mixed with starch the color changes dramatically to a dark black.