A recently formulated hypothesis relates the viscosity of polydimethylsiloxane (PDMS) additives to neutral, profoamant, or antifoamant performance in esters. At ten parts-per-million of PDMS addition, very low viscosity PDMS additives are hypothesized as neutral or ineffectual, due to solubility in the ester. Intermediate viscosity PDMS additives are profoamant due to increased surface activity attributed to more limited solubility of the partially miscible PDMS in esters, and Lewis acid-base interaction of the siloxaneester couple. High viscosity dispersed particulate polydimethylsiloxanes are antifoamant, inducing bubble collapse by a Griffith flaw mechanism. The foaming tendencies of an ester are related here to varying concentrations of PDMS of intermediate viscosities, i.e., 500, 1000, and 5000 mm2s−1(cSt), which are similarly neutral, profoamant, and antifoamant as concentration is increased. The hypothesis proposed is supported by the observations reported and provides a basis for a more complete understanding of observed foaming behavior.Presented at the 48th Annual Meeting in Calgary, Alberta, Canada May 17–20, 1993