The strength of Mo/Pt multilayers of varying thicknesses has been investigated using nanoindentation. Metallic composites with individual layer thicknesses ranging from 100 to 20 nm were fabricated. Of specific interest in this study was the strength of the nanocomposites after annealing in air at relatively high temperatures (475 A degrees C) since potential applications involve high temperature and oxidizing environments. Annealing causes significant losses in strength, with the highest losses corresponding to the structures with the thinner Pt layers. Annealing caused grain coarsening as well as loss of the continuous interface between the individual layers when the Pt thickness was less than 35 nm. Oxidation of the Mo layers occurred during annealing, causing an increase in the thickness of the Mo containing layers. The oxidation of Mo occurs in a uniform manner which results in an increase of the total film thickness while the layered structure is maintained. Deconvolution of the Mo 3d spectrum from X-ray Photoelectron Spectroscopy revealed several oxide species, and no Pt-Mo intermetallics were detected. The changes in microstructure are related to the changes in mechanical properties. Films with thinner layer thicknesses were stronger prior to annealing; however, they showed larger losses in strength after the thermal treatment. Structures with thicker Pt layers should be used when the multilayers are exposed to elevated temperatures.