The photophysics of fully and partially covered self-assembled monolayers (SAMs) of a quinquethiophene (5T) derivative have been investigated. The monolayers behave as H-aggregates. The fluorescence of fully covered SAMs is weak and red shifted, and the extinction is blue shifted as compared to that of single molecules. The fluorescence of partially. covered SAMs is dominated by that of single molecules on the surface. The extinction spectra are similar for fully and partially covered monolayers, which show that even the smallest islands are H-aggregates. The extinction spectra furthermore closely resemble those for 5T single crystals, which demonstrates that in oligothiophene crystals the intermolecular interactions within one layer molecules are stronger than the interlayer electronic coupling.