A major problem in the practical application of antifoams (substances used to avoid undesirable foam) is the gradual loss of their activity in the course of foam destruction. Several experimental methods are combined in the present study to reveal the origin of this phenomenon, usually termed as the antifoam "exhaustion" or "deactivation". A typical mixed antifoam, comprising silicons oil and hydrophobized silica aggregates of fractal shape and micrometer size, has been studied in solutions of the anionic surfactant sodium dioctylsulfosuccinate (AOT). The results unambiguously show that the exhaustion in this system is caused by two interrelated processes: (1) segregation of oil and silica into two distinct populations of antifoam globules (silica-free and silica-enriched), both of them being rather inactive; (2) disappearance of the spread oil layer from the solution surface. The oil droplets deprived of silica, which appear in process 1, are unable to enter the air-water interface and to destroy the foam lamellae. On the other side, the antifoam globules enriched in silica trap some oil, which is not readily available for spreading on the solution surface. As a result, the spread layer of silicone oil gradually disappears from the solution surface (process 2) due to oil emulsification in the moment of foam film rupture. Ultimately, both types of globules, silica-enriched and silica-free, become unable to destroy the foam films, and the antifoam transforms into an inactive (exhausted) state. The introduction of a new portion of oil (without any silica) on the surface of an exhausted solution results in a perfect restoration of the antifoam activity-reactivation of the antifoam. The experiments show that the reactivation process is due to restoration of the spread oil layer and to rearrangement of the solid particles from the exhausted antifoam with freshly added oil into new antifoam globules having optimal silica concentration. The results provide deeper insight into the mechanisms of antifoam action and suggest ways for improving the antifoam efficiency and durability.