A study has been made of the spray regime of sieve plate operation, and the transition to the froth regime. A simple fluidisation model for spray gives a simple criterion for the transition. The upper limits of spray operation are set by con­ sidering drop terminal velocities and limiting diameter - velocity relationships for stability. A further criterion for existence of spray is the form­ ation of a sufficient quantity of large drops, and the spray regime is shown to be coexistent with the orifice operation regime of pulsating jets which alone are able to generate the drops. Detailed study of single orifice operation by high-speed cine photography and pressure pulsation analysis has shown that there are at least six distinct regimes of bubbling and jetting according to the liquid depth and gas velocity, and each of these may be identified with a particular mode of sieve plate operation. The regimes of steady and pulsatine jetting are described by theoretical analyses which show how drop entrainment is produced from submerged orifices and how pressure drop is directly related to entrainment generation.