Thin films of poly(2-decyloxy-1,4-phenylene) (DO-PPP) are studied, especially in terms of their structure and properties. DO-PPP has thermal stability up to 384Â°C. The indium-tin-oxide (ITO)/DO-PPP/Al device emitted 425-nm blue light. The optimum thickness for the device is about 100 nm. For the ITO/DO-PPP (100-nm)/Al device, the EL quantum efficiency was about 0.01%. For the ITO/DO-PPP (100-nm)/Al device with a heat-treated polymer layer, the electroluminescence (EL) intensity clearly decreases with an increase of crystallinity. This result may be due to low carrier mobility in the crystalline region.