Abstract: Two caps are axially nested so that a smaller inner threaded cap is free to rotate within the outer larger protective cap and retained therein to form a security lock cap unit for gas supply tanks. It includes a locking means associated with the outer cap whereby the inner cap can be selectively engaged by the locking means to prevent its relative rotation, allowing the inner cap to be screwed on to a gas supply tank by rotating the outer protective cap and thereafter the inner cap is released by the locking means and retained in a disengaged position to prevent removal of the locking cap unit without a key to the locking means.