The effects of thermal annealing on the electro-optical and morphological properties of organic light-emitting diodes (OLEDs) containing a blend of poly[2-(2-ethylhexyloxy)-5-methoxy-1, 4-phenylenevinylene] and an electron-transporting material, 2,7-bis[2-(4-tert-butylphenyl)-1,3,4-oxadiazol-5-yl]-9,9-dihexylfluorene (DFD) are reported. Improvements in the efficiency were found to depend on the composition of the blend film. Devices with a higher concentration of the electron-transport material showed a smaller increase in efficiency following the annealing step. The thermal treatment is thought to result in the formation of an interfacial layer between the organic film and the aluminum cathode, improving the injection of electrons from this electrode. Blended-layer OLEDs containing a high DFD concentration (>=70%) retained their emission color following thermal processing.