The strength of Mo/Pt multilayers as a function of layer thickness has been investigated using nanoindentation. Structures with layer thicknesses varying from 20 to 100 nm were sputter deposited and the hardness of the films was measured using nanoindentation. It is shown that at this length scale the strengthening behavior of the films can be described by the confined layer slip model which has been proposed for other multilayered systems. Of specific interest in this study was the increase in the strength of the films after aging in ambient conditions with no apparent change in modulus. X-ray photoelectron spectroscopy revealed partial oxidation of the Mo layers in the aged samples which was associated with an increase in the residual compressive stress of the films measured by the bulge testing technique. The formation of molybdenum oxides in the Mo layer could account for the increase in strength through a second phase precipitation strengthening mechanism.