The nonlinear optical properties of a polydiacetylene, poly[1,4-(bis-3-quinolyl)-buta-1,3-diyne] (PDQ), in solution, were studied using the Z-scan technique with pulse trains at 532 nm. The results revealed a predominant slow accumulative process caused by thermal effects, with no indication of an electronic contribution. This calls for caution when analyzing nonlinear optical effects on polymer samples under resonant excitation. PDQ is amenable to Langmuir monolayer formation, and several layers may be transferred in optical-quality, Y-type Langmuir-Blodgett (LB) films if mixed monolayers with cadmium stearate (CdSt) are transferred onto solid substrates. Langmuir films were studied by surface pressure and surface potential isotherms, which indicated strong interaction between the two components. FTIR spectra of the mixed LB films confirmed the transfer of CdSt and PDQ, while UV-Vis spectra indicated a uniform transfer of PDQ during multilayer LB deposition. As in the case of most mixed LB films of polymers and CdSt, X-ray diffraction (XRD) results were close to those of pure CdSt films, pointing to CdSt domains in the mixed LB film. It is also found that the thin LB films are not suitable for use with the Z-scan technique, as the films are damaged when the near-resonant condition is employed. (C) 2003 Elsevier Science B.V. All rights reserved.