With a simple ionic model to quantify the ionicity in oxides and organic compounds, a common pattern of behavior is found for their conductivity and optical absorption. As the ionicity of these compounds decreases, the energy of optical absorption decreases, but they remain insulating, ionic compounds-until a critical ionicity, below which the optical gap vanishes, the oinic state becomes unstable, and interesting properties appear. In both classes of materials, just below this critical ionicity they are mixed valence and have metallic conductivity. Organic compounds with lower ionicity form neutral solids, while the corresponding oxides do not form stable compounds. In addition, there are some compounds of both types near the critical ionicity in which a phase transformation can be induced: neutral-ionic transitions in organics, and metal-insulator transitions in oxides. © 1992 Academic Press, Inc.