Foam tests and model experiments with sodium dodecylbenzenesulfonate solutions are performed to clarify how the foam stability and the foaminess are affected by several oils of different chemical structure. The foam tests show that 2-butyloctanol (2BO, branched alkanol) and isohexyl-neopentanoate (IHNP, branched ester) exhibit a significant antifoam. activity at concentrations as low as 0.005 wt %. n-Heptanol also acts as an antifoam, but at concentrations above 0.15 wt % due to its higher solubility in the surfactant solution. The model experiments prove that the antifoam activity of pre-emulsified oils is determined primarily by the barrier to drop entry, which controls the drop emergence on the solution surface. If the entry barrier is high (e.g., n-dodecanol and silicone oil), the oil drops remain arrested in the Plateau borders during the process of foam drainage, without being able to destroy the foam. Thus branched long-chain alkanols (like 2BO) and esters (IHNP) behave as active antifoams, because they combine the advantages of long-chain and medium-chain n-alkanols-low solubility and low entry barrier, respectively. No direct correlation between the spreading behavior of the oils and their foam breaking activity is observed. The effect of these oils on the foamability of the solutions is far more complex. At low concentrations (below and around their solubility limit) the oils reduce the dynamic surface tension of the solutions, facilitating in this way the formation of fresh surface and enhancing the foamability. At higher oil concentrations, however, the emulsified oil drops induce a coalescence of the foam bubbles during foaming and, as a result, the foamability of the solutions decreases. That is why the foamability is a nonmonotonic function of the oil concentration.