Colloidal gold adsorbed onto positively charged substrates forms a two-dimensional layer with a maximum coverage of 1 colloid/1000 nm2 as shown by optical absorption spectroscopy, small-angle X-ray scattering, and atomic force microscopy. Positively charged substrates are obtained by either chemisorption of end-group-functionalized silanes or by polycation adsorption. Bulk diffusion controls the gold adsorption; thus lower coverages can also be achieved. The optical spectra not only contain information on single colloids but also on their mutual interactions, as well as on the polymeric and solvent environments. Absorption spectra based on Mie and Maxwell−Garnett theory are calculated. The films remain stable on exchange of the external solvent. The local dielectric environment is shown to not only influence the position of the absorption band but also, more drastically, the intensity (by a factor of 2.5), in agreement with theoretical predictions (±15%). Particle aggregation induced by branched polycatio...