A clamping unit for containers at container treatment machines where, apart from a vertically movable lifting arm with a centering support for clamping a container from above, a motor for driving the lifting arm and a pneumatic spring against the resistance of which the centering support is mounted at the lifting arm so as to be movable to the top are provided. The lifting arms may be set to a desired vertical position independently of the movement or the rotational position of an associated rotary machine, and a clamping force of the centering supports may be easily and flexibly adjusted by applying a suited pressure to the pneumatic spring.