A reservoir cap to be positioned on a neck of a container at an aperture thereof, including a first element in which a chamber, closed by a pierceable wall, is provided to house a product introducible into the interior of the container, and a second element including a piercer for piercing the pierceable wall. The first and second element being axially movable relative to each other, such that the movement causes piercing of the pierceable wall, with consequent introduction of the product into the container. The first element includes a first connector for connection to the container, which cooperate with an external portion of the neck of the container, and second connector for connection to the second element. The second connector including a thread cooperating with at least one tooth engaged in it.