The structure of NH4I has been studied by energy-dispersive x-ray diffraction at pressures between 1.3 and 68 GPa, and by optical-absorption techniques at pressures up to 75 GPa. The solid has a CsCl-type structure over the pressure range studied. A first-order Birch equation of state was used to calculate the bulk modulus and its first pressure derivative. The band gap drops from 5.5 eV at atmospheric pressure to 0.86 eV at 74.8 GPa and the extrapolation of data suggests metallization in the neighborhood of 1.2 Mbar (120 GPa). © 1987 The American Physical Society.