Highly crystalline g-C3N4 nanoparticles have been synthesized from molecular precursors via polycondensation, using mesoporous silica with different pore sizes as host matrices. The particles isolated after removal of the host matrix (see Figure) are characterized using electron microscopy and various spectroscopic techniques. Structural and electronic size effects are identified by comparing nanoparticles that are 5, 13, and 60 nm in diameter.