Electroluminescent devices from binary blends of conjugated polyquinolines are fabricated and used to systematically investigate the mechanisms of efficient electroluminescence (EL) in multicomponent conjugated polymer system. The roles of energy transfer, excited state complex formation, charge transport and trapping, miscibility and phase separation, and spatial confinement on EL efficiency can be directly probed by a judicious choice of binary blend components. Large enhancement of EL efficiency and device brightness was observed in some conjugated polymer blend systems compared to the component homopolymers. For example, binary blends of poly(2,2'-(2,5-thienylene)-6-6'-bis(4- phenylquinoline)) and poly(2,2'-(biphenylene)-6,6'- (4-phenylquinoline)) are found to exhibit EL quantum efficiency of up to a factor of 30 enhancement at 200 cd/m2 luminance levels compared to the components. EL enhancement in this and other polymer blend system is shown by electric field-modulated photoluminescence spectroscopy and other experiments to be due to improved radiative electron-hole recombination efficiency facilitated by spatial confinement of excitons in the blends.