The formation of H-aggregates as a function of solution, substrate and ambient variables is considered for the merocyanine dye 3-acetyl-5-12-(3-ethyl-2-benzothiazolydene) rhodanine. Colloidal semiconductor particles are shown to be a powerful tool to control the size of the aggregates. In water the blue shifted absorption band has been assigned to a dimer. Its spectrum has been isolated and the thermodynamical variables derived for the dissociation reaction are: ΔrG0=21.16 kJ/mol, ΔrH0=32.36 kJ/mol and ΔrS0=37.24 J/mol K. Exciton band absorption maxima for aggregates in solution and at the water-TiO2 and water-Al2O3 interface, respectively, have been correlated to the aggregation geometry using the extended dipole model in conjunction with crystallographic data. Microcrystals showing a hypsochromical shift in the absorption band have been produced within the pores of nanocrystalline semiconductor films. The calculation shows that these aggregates are needle shaped and are composed of about 2250 monomer units. A broad emission band appears when the organic molecules assemble in a head to tail stacking geometry which could be attributed to excimer fluorescence. It is not quenched by charge injection into TiO2 and indicates the existence of dislocations within the merocyanine stacks.