A method of dissolving a gas in water comprising directing an aqueous stream to rotate in a downwardly moving spiral stream from a high downward velocity to a lower downward velocity and diffusing a gas into the aqueous stream so as to produce small gas bubbles which are rotated in a generally spiral path by the aqueous stream thereby providing a long flow distance over which the bubbles have increased contact time with the aqueous stream to facilitate dissolution of the gas therein and in which rising of the gas bubbles is progressively opposed by the increased downward velocity of the higher portion of the aqueous stream.A vessel for practicing the method can have a vertical axis and be circular in horizontal section for most of its height; have upper and lower ends; include a conical portion with the apex at the upper end of the vessel; have an aqueous stream inlet tangential to the vessel communicating with the vessel interior space near the vessel upper end so that an aqueous stream fed to the vessel through the inlet rotates in a downwardly moving spiral stream from a high downward velocity in the upper portion of the vessel to a lower downward velocity in the lower portion of the vessel; and have a gas diffuser for diffusing a gas into the aqueous stream.