Abstract: A lockable gas cylinder cap system comprised of hinged upper and lower cap portions and a lockable equipment or parts holding box removably attached to the lower cap portion by a T-shaped connecting member that is slidably received in a downwardly extending slot provided in edge and sidewall of the lower cap portion. The box is provided with a hingably secured lid that includes an outwardly extending locking tab member that overlies the T-shaped connection when the lid is closed so as to be retained between the upper and lower portions of the cap when they are closed together.