A device for filling a high-pressure CO.sub.2 cylinder with liquid CO.sub.2 while venting gaseous CO.sub.2 comprises a two-compartment chamber that is installed in the top opening of the cylinder. A first dip tube connected to one compartment extends to the bottom portion of the cylinder and a second dip tube connected to the other compartment extends to the upper portion of the cylinder. Each compartment has a port for flow into or out of the compartment. Liquid CO.sub.2 introduced into the compartment with the first dip tube will flow into the cylinder and cause gaseous CO.sub.2 to exit through the second dip tube and the other compartment. When liquid CO.sub.2 appears in the exiting stream, the introduction of liquid CO.sub.2 is stopped. In a common embodiment, a conventional CO.sub.2 valve has the first dip tube connected to the bottom thereof. A street tee with one end connected to the second dip tube is screwed into the top opening of the cylinder. The CO.sub.2 valve is screwed into the other end of the tee, the first dip tube extending with annular clearance through the second dip tube into the cylinder.