Blue and green light-emitting electrochemical cells (LECs) based on a ladder-type poly(paraphenylene) ( m-LPPP) are described. These LECs are characterized by an extremely low onset for light emission (between 2.2 and 2.7 V in reverse direction for blue emission) yielding an external quantum efficiency of approximately 0.3% and a brightness of about 250 cd/m 2 at 10 V. The LECs based on m-LPPP show a fast pulse response below 30 μs, which is ascribed to restricted motion of the ions in the LEC under pulse operation. Light-emitting diodes based on conjugated polymers (PLEDs) can be realized with promising efficiencies over the entire visible spectral range.1-6 The most attractive potential application of PLEDs is their use in emissive flat panel displays. In order to realize PLEDs with operating lifetimes which fulfill the values required for industrial applications, efficient devices with low operating voltages are necessary. Low operating voltages are more easily realized with small bandgap conjugated polymers, because the mismatch between the energy bands of the polymers and the work functions of the electrodes1 is smaller than for wide bandgap polymers. Thus, it has proved to be easier to realize red-orange and green PLEDs with low operating voltages and high efficiencies. 1,2 However, the fabrication of blue PLEDs with low turn-on voltages and sufficient lifetimes is absolutely necessary to realize multicolor flat panel displays. Moreover, one of the most promising ways to produce multicolor displays is based upon blue PLEDs, and the realization of red and green light emission through down-conversion.7-9