A method and apparatus for sensing the concentration of a gaseous sterilant in a sealable enclosure. An indicator is provided that has a chemistry such that the indicator changes color when exposed to vaporized hydrogen peroxide (VHP). The chemistry is adapted to react when exposed to a specific minimum concentration of vaporized hydrogen peroxide for a specific minimum period of time. In this manner, it can be visually determined whether articles (e.g., medical instruments and like devices) located within the sealable enclosure have been exposed to a minimum threshold of vaporized hydrogen peroxide.