Betaines (a particular class of amphoteric surfactants) are commonly used as foam boosters in various products to improve their foamability and foam stability. Foaming media often contain dispersed drops of of silicone or hydrocarbon oil, which act as foam destruction agents (antifoams). A complementary set experiments on foams and foam films stabilized by an anionic surfactant, sodium dodecyl-polyoxyethylene-3-sulfate (SDP3S), or by mixtures of SDP3S and Betaine, is performed in the present study to clarify the mechanisms of: (1) foam destruction by silicone oil drops, and (2) foam boosting effect of betaine in the presence of oil. The experiments show that foams stabilized by SDP3S are very stable in the absence of oil, while they are unstable and decay with time in the presence of oil (antifoam effect of the oil). The introduction of 40 molar % betaine in the mixture leads to complete foam stabilization in both cases-with and without oil (foam boosting effect). Notably, the size of the oil droplets hat; a significant effect on the foam stability-a substantial amount of silicone oil can be introduced without deteriorating the foam stability, if the drop diameter is below ca. 5 mu m. Optical observations of the process of foam film thinning show that the oil drops leave the films without destroying them in all of the studied systems (stable and unstable) relatively soon after foam formation-typically, within less than a minute. The foam destruction occurs at a later stage of the foam evolution, when the oil drops are compressed by the walls of the narrowing Plateau channels as a result of liquid drainage from the foam. Surface and interfacial tension measurements show that variations in the values of entry, E, spreading, S, and bridging, B, coefficients cannot be used to explain the observed foam boosting effect of betaine. On the other hand, direct measurements of the critical capillary pressure leading to drop entry demonstrate that the barrier to drop entry is much higher in the presence of betaine. The data unambiguously show that the main role of betaine as a foam booster in the st;died systems is to increase the barrier to drop entry, which leads to suppressed activity of the silicone oil as an antifoam. The obtained results provide deeper insight into the foam boosting effect and suggest some clues about the properties which an efficient booster should possess.