The optical properties of periodic arrays of subwavelength apertures in metal films on GaAs substrates are studied. Specifically, geometric and material losses for these plasmonic structures are characterized using angular dependent transmission, normal incidence reflection, and angular dependent diffraction experiments, in addition to a crossed-polarizer transmission experiment. The optical properties of the samples as a function of engineered material losses are studied. Using this comprehensive approach to the characterization of the plasmonic structures, we are able to identify and isolate specific loss mechanisms, as well as identify the effect of free carriers on the optical properties of the structures.