Abstract: A pumping system for filling pressurized gas cylinders from a relatively lower pressure, liquified supply of that gas, for example CO.sub.2, by means of a system comprising a CO.sub.2 /air-driven motor driving the liquified gas into the cylinder. The system is primed until liquid begins to run. Thereafter the liquid is driven by the CO.sub.2 /air-driven motor into an empty cylinder. A particularly efficacious longitudinally reciprocating plunger pump for use in the system is disclosed (FIG. 4). The "air" driven motor is preferrably driven by CO.sub.2 vapor, particularly where CO.sub.2 is the liquified gas being pumped or where hazardous or toxic liquified gases are being pumped.