Soluble conjugated polymeric nanoparticles are synthesized by Suzuki-type polycondensation of two monomers (Ax + By, x>2, y≥2) in the channel of ordered mesoporous silica-supported carbon nanomembranes (nanoreactors). These synthesized soluble conjugated microporous polymers (SCMPs) exhibit uniform particle-size distributions and well-controlled particle sizes. The control of particle size stems from the fact that the polycondensations exclusively take place inside the mesochannels of the nanoreactors. Photoluminescence studies show that polymeric nanoparticles with tetraphenylethene and pyrene substructures are highly fluorescent. The combination of both physical stability and processability offered by the soluble polymeric nanoparticles makes them particularly attractive in light emitting and other optoelectronic applications.