The mechanism of the antifoam deactivation process was investigated. The antifoaming capacity of various antifoam oils and oil + hydrophobic solid particle mixtures was measured with single foam films and with bulk foams. When the antifoam became inactive, there was monolayer coverage at the solution surface, The antifoam deactivation results could be explained by the emulsification of part of the spread oil on the film during film rupture into ineffective, small oil drops, In the case of mixed antifoams, the film breaking capacity extrapolated to zero film area was equal to the specific foam breaking capacity of these antifoams only if the net area of the foam films was taken into account, Moreover, the film/foam breaking capacity of mixed antifoam (9.4 +/- 2 m(2)/mg) was several times greater than the capacity of the oil component (2.3 m(2)/mg), To explain this difference, a mechanism was suggested in which the spreading oil drags the antifoam lenses and the oil coverage is still low when the antifoam lenses bridge and rupture the him. The preemulsified antifoam had lower activity and film/foam breaking capacity (durability) than the antifoams which were layered on the surface of the solution as a bulk.