Metastable alloy films of a number of binary alloy systems with limited terminaI solubilities were prepared by simultaneous vapor deposition of the two components onto a cold substrate. The resulting structures are either crystalline solid solutions (of concentrations higher than the terminal solubility) or amorphous solutions. Several examples for both types of structures are described. The conditions for occurrence of the amorphous structure are found to be a difference of atomic radii of the components in excess of 10% and a sufficiently low substrate temperature. The composition range of the amorphous structure increases with increasing difference of the atomic size of the components. Amorphous structures generally decompose at about 0.3 Tm (where Tm is the average melting temperature), while metastable crystalline structures decompose near 0.45 Tm. Thus, for many of the alloys studied, the metastable structure persisted to room temperature and above.