Single walled carbon nanotubes (SWNTs) are reservoirs of gases as they can adsorb on their walls as well as retain gas molecules in their hollow cylindrical interior. This has important applications for example in fuel cell technology for hydrogen storage, as a gas sensor for realization of artificial nose, etc. Storage of gases by carbon nanotubes have been recently investigated by monitoring changes in their thermoelectric power and electrical resistivity due to their interaction with gas molecules. In this paper we present a gravimetric study of interaction of gas molecules with isolated SWNTs using a micromachined ultrasensitive quartz crystal microbalance (QCM). The adsorption and desorption of gas molecules with different molecular weights from carbon nanotubes revealed that changes in resonance frequency and quality factor of the resonating crystal scale as approximately M0.45, where M is the mass the of the gas molecule as compared to M1/3 dependence observed in case of changes in thermoelectric power and electrical resistance for thin films of the carbon nanotubes. The use of QCM enables room temperature probing of gas interaction with isolated single walled carbon nanotubes. Specific interaction of gases with carbon nanotubes on QCM provides potential application of the device as a gas sensor.