Abstract: Liquid in which oxygen-containing gas is to be dissolved is pumped through a main and distributed into faster-moving subsidiary streams. Each subsidiary stream passes through a turbulence-inducing means and air or oxygen is introduced into the turbulent region of alternate streams. The relative proportions of air to oxygen introduction are varied so as economically and efficiently to oxygenate the liquid. The gas-containing liquid passes through nozzles back into the volume of the liquid at sufficient velocity to cause further turbulence, making gas bubbles shear into smaller bubbles and also agitating the body of liquid.