Abstract: A transfer film comprised of an inert backing coated with a transfer layer comprising a mixture of a resin, a filler, a magnetic pigment, a nondrying oil and an oil soluble dye provides security against fraudulent alteration of Magnetic Ink Character Recognition (MICR) symbols. Upon impact this film acts like an ordinary MICR transfer film: a portion of the transfer layer transfers to a document surface forming a magnetically readable character image. After transfer, the nondrying oil contained in the transferred layer begins to diffuse into the matrix of the document paper. The oil carries the visible oil-soluble dye. Soon an image of the MICR characters appears on the reverse surface of the document. If any of the characters do not show through on the reverse surface as a visible dye image, alterations have been made to the MICR characters.