An differential expansion system and method for balloon buoyancy control which includes a zero-pressure envelope and a superpressure envelope. A gas transfer device is positioned between the envelopes to facilitate the transfer of lift gases that are lighter than air between the envelopes. The superpressure envelope and the gas transfer device can be located inside the zero-pressure envelope. Alternatively, the zero-pressure envelope, the superpressure envelope, and the gas transfer device can be located inside a zero-pressure envelope.