Phospholipid dispersions that undergo a multilamellar vesicle (MLV) to unilamellar vesicle (ULV) transformation at a critical temperature T* form a single bilayer spontaneously and rapidly at the air-water interface. The "surface bilayer" rapidly covers the entire air-water surface, yielding a surface tension similar to 70 mN/m. This indicates that the outermost layer of the surface bilayer is its hydration layer. At 100% relative humidity (R.H.) the surface bilayer is stable indefinitely but becomes unstable, forming a monolayer, when the R.H. is reduced. The rate of breakdown to monolayer varies inversely with the R.H. The rapid formation of the surface bilayer is a manifestation of the superposition of surface forces on the MLV-ULV transformation that occurs simultaneously within the bulk dispersion at T*. Surface forces act primarily to accelerate the MLV-ULV transformation.