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The gas transfer in aeration systems is broken into two processes: gas transfer at the bubble interface and gas transfer at the water surface. Experiments were conducted to separate these two sources of dissolved gas. Oxygen absorption was measured in a laboratory tank with air being diffused through a porous diffuser and then with nitrogen gas being diffused. The combination of these experiments along with reformulation of the theoretical transfer equation permit separation of the gas transfer at the water surface and in the rising bubble plume. Estimates of the exchange coefficient for the plume and surface are given.