Elastic moduli of 50-250 nm thick Al-50 at Cu film alloys deposited by thermal evaporation on Kapton substrates and postformed by thermal diffusion are investigated. Formation of the Al 2Cualloy phase was confirmed by X-ray photoelectron spectroscopy (XPS). Surface morphology was examined by atomic force microscopy (AFM) and scanning electron microscopy (SEM) before and after tensile mechanical testing. Force-strain curves of the Al-Cu alloy were obtained by subtracting the effect of the force-strain Kapton curves from the corresponding curves of the Al-Cu/Kapton system. A reduction in the elastic modulus of the Al-Cu alloys from 106.1 to 77.8 GPa with the increase of alloy thickness was obtained. Measured elastic moduli were between the reported bulk modulus for Al and Cu. Reductions in the surface roughness and increments in the grain size were measured after tensile testing of the Al-Cu alloys.