The possible origins of the reduced foamability of dilute solutions of chemically modified ethoxylated nonionic surfactants are discussed. First, the organization of the modified molecules at the interface is different from that of the standard ethoxylated compounds. After diffusion from the bulk to the interface, the molecules undergo a rearrangement to take a folded conformation at the surface. This second process, observed in the dynamic surface tension measurements is much slower than the first one. Therefore, during the foam formation, the interfaces of freshly formed films are not sufficiently covered with the surfactant and, hence, are badly protected against film rupture. Moreover, the modified surfactants have a smaller surface viscoelasticity, thus enhancing the thinning and the breaking of foam films.