Excessive foaming in multistage flash desalination plants can give rise to saltcontaminated distillate and reduced operating efficiency. Plant operating variation: in pressure, temperature, flow, and seawater feed canposition and alkalinity can perturb vapor/liquid equilibria, resulting in increased levels of foam. Since such foaming is not always controlled by mechanical means, add i control foaming level _ tives are often needed to In addition to controlling foam, a useful additive must not interfere with heat transfer or with the performance of the scale control add tive; it must not contain steam-volatile components that can contaminate the distil ate; it must be stable at MSF operating temperatures and residence times; and it should be compatible with thf antiscalant feed solution. Antifoam performance data for over 60 candidates in 7 chemical classes were determined according to these criteria. The most effective structural class was found to be the polyglycol monoalkyl ethers. Foam control effectiveness within this class was shown to be a function of ethylene oxide content and alkyl chain length. The most effective additive found in this sttidy is offered camnercially as FLOCON Antifoam AF-4. FLOCON AF-4 has been used successfully in high-temperature MSF plants.