X-ray diffraction studies to 73 GPa show that CsI becomes orthorhombic at about 56 GPa. Infrared absorption studies show the band gap dropping from 6.4 to 0.36 eV at 62 GPa. At 65 5 GPa, CsI becomes metallic and reflects in the red, which we attribute to the plasma edge being in the visible. The interatomic distances of the nearest like neighbors are reduced from the zeropressure value by 28% at metallization where CsI has a pseudo tenfold coordination. © 1985 The American Physical Society.