Some resuits from an experimental investigation into the mechanism of bubble nucleation in the flash boiling of water are presented. Conditions similar to !hose found in polyphosphate dosed MSF plants have been studied. The experiments suggest that the naturally occurring nucleation sites in sea water give an effective site radius of the order of 15-20 microns. Such sites with the help of dissolved gases would be effective only over the top quarter of the flash range in a typical plant. Diffusion of gas towards the sites is thought to be responsible for the time delays of the order of seconds between decompression and nucleation. SY (BoLS D -Muss diffusion coefficient. cm'/scc d -Nucleation suppression depth for infinite site size, in. J 20/l -Nucleation suppression depth for 20/t site size, in. d 4op -Nucleation wppression depth for 40/c site size. in. (I lOO,u -Nucleation suppression depth for 1001' site size. in. 0, ppm -Dissolved oxygen content, ppm ap, p44 P co: PJ R CE R CT TO -Interstage pressure difference. bars -Partial pressure of air. bars -Partial pressure of carbon dioxide, bars -Hydrostatic pressure, bars -Equilibrium radius in absence of gas diffusion, microns -Equilibrium radius with gas diffusion. microns -input stage temperature, "C -Flash equilibration fraction -Deaeration equilibration fraction -Thermal diffusivity, cm'/sec -Surface tension. bar microns 21s W.