We report the self-assembly of ultrasmooth AuxAg1−x nanoparticles with homogeneous composition via pulsed laser-induced dewetting (PLiD). The nanoparticles are truncated nanospheres that sustain unique plasmonic features. For the first time an electron energy loss spectroscopy (EELS) study elucidating the size and composition effects on the plasmonic modes of truncated AuxAg1−x nanospheres is carried out. EELS characterization captures a linear red-shift in both bright and dark modes as a function of the atomic fraction of Au and a progressive red-shift of all modes as the size increases. The results are interpreted in the context of Mie theory and electron beam simulations. Armed with the full plasmonic spectrum of the AuxAg1−x system, the truncated spheres and their ordered arrays synthesized via PLiD have promise as elements in advanced photonic devices.