Effects of laser irradiation on substrates during deposition (laser-assisted deposition) of 10-(1-pyrenyl)decanoic acid (PyC(10)) under various conditions (laser fluence of 0.05 and 0.5 mJ cm(-2) and substrate temperature of 246, 287 and 313 K) were investigated by measuring in situ fluorescence spectra during deposition. Laser irradiation effects on PyC(10) films prepared by normal deposition (irradiation after deposition) were also investigated for comparison. Laser irradiation after deposition caused change of the local structure of fluorescent sites, but not a change of the main pyrenyl aggregate structures responsible for absorption spectra. Fluorescence spectra of films prepared by laser-assisted deposition with a fluence of 0.05 mJ cm(-2) were different from those of normally deposited films, while absorption spectra of both the films were identical with each other. For laser-assisted deposition on hydrophilic substrates with a fluence of 0.5 mJ cm(-2), both fluorescence and absorption spectra were different from those of normally deposited films, indicating that a new aggregate structure of pyrenyl chromophores was induced by laser-assisted deposition with a high fluence. Formation of the new aggregate was considered to be attributable both to interaction between the excited and ground state molecules and to thermalization energy from the excited state. In the case of laser-assisted deposition on hydrophobic substrates, the new aggregate of pyrenyl chromophores was not induced even under irradiation with a high fluence of 0.5 mJ cm(-2).