A flow of a gaseous mixture of first and second substances at a desired ratio is provided to a storage container or point of use. Each of the first and second substances has the property of existing only as a gas when it is maintained at one atmosphere and 0° C. A vessel contains the first substance in gas phase only and a second substance in both liquid and gas phases. The second substance is bubbled into the liquid phase of the first substance thereby resulting in the gas mixture in the vessel headspace. The interior vessel temperature, the pressure at which the second substance is bubbled, and a flow rate at which the gas mixture exits the vessel are all maintained at levels sufficient to provide the desired ratio.