The thermal and mechanical stability of molybdenum and amorphous silicon (Mo/a-Si) optical multilayers (3 and 4 nm nominal thickness of Mo and Si) at 316 degrees C were studied by annealing experiments. Growth of amorphous Mo-Si interlayers with a stoichiometry of 1:2 was observed at the Mo/a-Si interfaces. In addition, residual stresses significantly changed in the crystalline Mo and amorphous Si layers with annealing. High resolution electron microscopy, selected area electron diffraction, and X-ray diffraction of the crystalline Mo revealed that tensile stresses increased from 2 to about 10 GPa in the lateral direction (parallel to the interface plane). The compressive strains that developed in the vertical direction (perpendicular to the interface plane) are consistent with Poisson’s ratio. Laser deflectometer measurements of thicker (0.1 mu m) amorphous silicon layers may indicate compressive-stress relaxation in the amorphous silicon with annealing, consistent with other investigations. Overall, the residual stress in a 40-bilayer film changes from about -0.5 to about +1.5 GPa. Structural transformation after relatively short annealing times at the interfaces in the thin amorphous Mo-Si interlayers may rationalize increased tensile strains in the Mo layers.