The effect of the annealing atmosphere on the magnetic properties of a rapidly quenched amorphous soft magnetic alloy Fe81B13Si4C2 was demonstrated by the examples of heat treatments in a vacuum and in ail and explained by the interaction of iron atoms present in the surface layer of the ribbon with water vapors. The anisotropic oxidation and hydrogenation in the surface layer of the ribbon related to the oxidation reaction generate an in-plane pseudo-uniaxial tension, which affects the magnetization distribution in the ribbon, the domain width, the pinning of domain walls, and, therefore, the level of magnetic properties. The conclusions obtained are also confirmed by a direct study of the effect of water on magnetic properties.