Nanocrystalline films of TiO2, Al2O3, and ZrO2 were used as hosts for the merocyanine 3-acetyl-5-(2-(3-ethyl-2-benzothiazolidinylidene)ethylidene)rhodanine (Mc2) to scrutinize templating effects in the accommodation of the dye within their porous network. Composed of interconnected mesoscopic oxide particles and pores, these films are transparent, allowing for a straightforward application of transmission spectroscopy to unravel the optical features of the incorporated dye species. Apart from H-aggregation, the formation of two different types of two-dimensional assemblies was witnessed yielding red-shifted absorption bands which were identified as J-aggregates, one showing Davidov splitting, the other having a single exciton band. The herringbone packing of the dye molecules in the layered structure of Mc2 sodium salt octahydrate single crystals was taken to model the double-banded J-aggregate structure. On mesoscopic hydroxylated TiO2 anatase films, the structure of the Mc2 assemblies is controlled by the texture of the highly porous substrates as well as their surface charge. Furthermore, it responds in a striking fashion to the presence of solvent in the ambient to which the films are exposed. Double-banded (herringbone structure) and single-banded (parallel alignment of the dye) absorption spectra can thus be obtained. The role of solvent is to stabilize one particular aggregate geometry through intercalation into the Mc2 aggregate. Electron injection into TiO2 from both types of J-aggregates is observed. Laser flash photolysis experiments show that energy transfer from the monomer to the J-aggregate is operative prior to charge injection. On hydroxylated Al2O3 and ZrO2 surfaces Mc2 undergoes physisorption and formation of H-aggregates exhibiting a blue-shifted absorption with regard to the monomer spectrum. Contrary to the results obtained for TiO2 substrates, aggregation is hardly influenced by solvent in the ambient. In particular no J-aggregates can be formed on bare Al2O3 and ZrO2 substrates. However, when the porous films are impregnated with concentrated hydroxide solutions, H- as well as J-aggregates are formed in humid air. At high humidity, due to the hygroscopic salt coating, the pores are completely filled with water, leading to the precipitation of the dye molecules forming H-aggregates. At lower humidity an air-water interface builds up within the pores an da double-banded J-aggregate spectrum appears. The spectrum is almost identical to the one measured on Mc2 sodium slat octahydrate single crystals with a layered organic-inorganic structure. Resonance fluorescence originating from the energetically lower exciton band and internal conversion from the higher to the lower exciton band take place.