Molecular aggregate formation in Langmuir-Blodgett films of dye molecules has been investigated, Mixed layers of 1-pyrenebutanoic acid (PHA) with cadmium stearate/stearic acid and pyrene with cadmium stearate/stearic acid have been deposited onto quartz substrates. Using the time-correlated single-photon counting technique and fluorescence spectroscopy, evidence for dimer formation in PHA multilayers and excimer formation in pyrene films was obtained. The different types of aggregates are attributed to structural differences between the pyrene and PHA mixed films.