Abstract: A gas compressor has at least one closed container with gas inlet means for filling the container with a gas to be compressed and gas outlet means through which compressed gas can leave the container. When the container has been filled with gas to be compressed, a fluid is forced into the container to compress the gas therein and force it out of the container. After the container has been filled with fluid and the gas forced therefrom, the fluid is drained while gas again enters and fills the container. The output of the container is generally connected to a gas storage tank and the cycle of filling the container with gas and then expelling it is repeated until a desired pressure of gas is obtained in the storage tank.The compressor is particularly suited to filling a storage tank, such as a natural gas storage tank in a vehicle, over an extended period of time of several hours.