Plasmonic materials offer the extraordinary property of optical field confinement, and find emerging applications in sensing, optical computing, sub-wavelength imaging among others. Developers wishing to exploit this phenomenon however, are limited by the sparse availability of reliable, experimentally-derived optical constant data (n & k) for the purposes of optical simulations. Typically, computational simulations employ values from Johnson & Christy’s tabulation or Palik’s handbook which are known to contain inaccuracies and are also limited to simple material compositions. A database of experimentally-derived optical constants for more complex materials therefore has value. We present the high-throughput synthesis of plasmonic alloys and rapid characterisation of their optical properties as a function of both composition and wavelength. The variance in refractive index and extinction coefficient show correlation with lattice parameter, crystalline phases of intermetallic compounds and other quality factors that would not be taken into account with a traditional effective medium approximation (weighted averaging of n & k with composition). Moreover, ellipsometry of compositional gradient thin films which are 1) metallic, 2) optically thick, and 3) smooth and highly reflective is an effective method for rapidly assessing the optical constants of complex plasmonic alloy systems.